@mirimouse: yes, I also think by writing, which is why I had most of this already typed out as preparation for my conversations with my husband. 
Some of these reasons are more rational than others, but despite probably 2 years of trying, I can’t seem to shake the desire.
In no particular order…
I have ALWAYS been a kid person, and particularly a baby person. I was a babysitter at 11 and all throughout high school and college. I even worked in a preschool as a sub for a while. Bottom line. I love kids.
I feel like I missed out on getting to fully appreciate the infant stages because of the stress of having 2 infants at once. I by no means think that having two 4 year olds and an infant will give me the same experience as having only one infant by himself, but I do truly believe it will be easier than two infants at once. It is the closest option I have to getting that experience.
One thing that is very emotionally challenging for me about having twins is that every first is also a last. Yes, they may not walk on the exact same day, but within a few weeks they have both typically mastered something (particularly during the baby stages) and so I don’t really have the same anticipation of upcoming milestones or interesting experiences. (The first time to the swimming pool for one is probably the first time for the other as well.) Plus, with a first child you are sort of going it blind for the most part, but after you’ve been through it you have experience that would be exciting to get to use again. Another related factor is that with twins going through similar things at similar times, if some experience gets screwed up that is sort of it. An example from our family: potty training was ruined by huge conflicts with our preschool (yeah I know what you are thinking, who likes potty training? Well I was having fun with it until I had to try to convince the preschool to get involved), and I feel almost ill when I think about it because I am so angry (still! and the bulk of the main conflict was months ago). I don’t think I will ever be able to think about it without being so angry. But if I were to get another chance, I would be able to use my prior experience to avoid having the same problems as we had the first time around, and maybe heal some of the bitterness about this first attempt. And yes, I realize that no matter how many children you have, you will always have a last for everything, but there is something about the first and last happening at essentially the same time that is very difficult for me.
I was an exchange student for a year in high school and I joined a family of 5 siblings. The 6 of us had so much fun together (and actually still do, 15 years later! since my husband and I moved to [name_u]Sweden[/name_u] and now live about an hour away from that family). Of course there was conflict sometimes, but on the whole the chaos of the large family was thrilling and something I still enjoy when we all get together to this day. I also got a sort of a taste of this growing up because my mom has 9 siblings so extended family gatherings were always exciting.
I think one of the most amazing things a woman’s body can do is nurture a life. Despite an uncomfortable twin pregnancy with the last two months on modified bed rest I loved trying to conceive, becoming pregnant and the haze of the boys’ infancy. The past 3 years have been some of the happiest of my life. While I fully recognize that the pregnancy and infancy are short term aspects of the decision, they are hugely intense and influential aspects of motherhood and I can not simply write off my desire to go through them again as invalid.
We have started hanging out more with families with babies and I have seen how my boys react to them. It is adorable. Having a baby around brings out some of my favorite characteristics in my sons: generosity, kindness, thoughtfulness, helpfulness etc… The pride I see in their eyes when they help out younger kids makes me happy. They like filling the role of the older, more capable person in a relationship, something they haven’t had much opportunity to do in their short lives thus far.
While I am not naïve about siblings and their pension for jealousy, I do think that my existing children could benefit from the experience of being the older brothers. (And yes, I worry about how a third child would feel with older twin siblings, but every individual has such unique personalities it is impossible to say whether that would be a problem or not. My twins are so different my husband and I have joked half seriously that we hope they stay friends later in life. Who knows, one of them could really bond to his little brother or sister…)
Another reason I would like a third child is the chance of having a daughter. Horrible, I know, but hear me out. My husband and I both love the idea of a daughter, but if we ended up with another son we would also be happy. Each person is an individual with a unique set of interests and characteristics so it is not so simple as all sons are like this and all daughters are like that. I could not love my existing boys any more than I do, and would have the same love for any future sons or daughters. When I was pregnant the first time I kept copious notes about everything I was going through. At our 18 week scan, when we found out they were both boys, we were both excited, but I noticed that I stopped writing about my pregnancy. Only later I realized that I had been writing for my daughter so she would have a sense of my experience when it came to be her time should she choose to have children. There is a deep connection between a parent an any child, but I think the nature of that connection can be different depending on the gender of the child. Not any more or less for any gender, but just the potential to be different and I would like the chance to have a connection with a daughter. (That said, I am not going to continue trying until I get a daughter. Three kids would be a stretch for us, so I recognize that if we tried again and got a boy I think I would accept that my chances would be over and I would pour all my love and affection into my 3 boys. I would like another chance for a girl, if that makes sense. )
We had our children an ocean away from our families and having a third child would give us the option to potentially raise a child closer to his or her grandparents. I think it would mean a lot to our parents. Again, not a complete reason to bring a child into the world, but something we have thought about.
I have come to realize that family is my most important priority. I am so grateful that I was able to be home with our children for so long after they were born (14 months) and that we live in a society where we are able to earn a living wage with me only working half time. The money is not as important to me as time with family. Of course, I want to have enough to feed and clothe my children and have enough to splurge sometimes on travel or a new toy, but we are definitely not ‘stuff’ people. (I totally recognized what you were describing about ‘leave behind/ sell/ pay to cart it along’ mirimouse!) Of course finances would be tighter with 3 than with 2, but in my mind the love and joy and we would gain from another family member would provide a richer experience than the extra money would have. Our biggest financial worry is college tuition…
When I try to look at it rationally I see lots of reasons not to have a third child, but I feel like I will regret it for the rest of my life if I don’t have the opportunity to have another kid.
OK, novel finished.
And I still really want to hear other people’s experiences grappling with this issue!