Okay, another thought, I also love Lucy, one name I always liked nearly as much was Cecily.
I think of the youngest (and kindest, sweetest little thing) daughter in Tales from Avonlea, or even Cecily Mary Barker the artist of all the beloved flower faires… which seems to fit your vibe a bit atleast.
Marigold, Jane & Cecily 
Others… Felicity, Blythe, Bronwyn, Elora, Isla, Aurelia, Willow, Luella, Martha, Freya, Poet, Anne, Gwen/Gwendolyn/Guinevere, Joy ?
I just want to stick up for this category of names a little! Maybe you should take a second look at them. The way I view these three names is they match the length of [name_f]Marigold[/name_f], and are a more classic style like [name_f]Jane[/name_f]. I feel they bridge the two perfectly! [name_f]My[/name_f] favourite would be
[name_f][/name_f][name_f]Marigold[/name_f], [name_f]Jane[/name_f] & [name_f]Annabelle[/name_f]
[name_f][/name_f]I like that all three have a “root” perennial classic. [name_f]Mary[/name_f], [name_f]Jane[/name_f] & Ann/Anna! They feel very cohesive! I also like that [name_f]Jane[/name_f] is in between the two longer names.
[name_f]Alice[/name_f]
[name_f][/name_f][name_f]Celia[/name_f]
[name_f][/name_f][name_f]Elsie[/name_f]
[name_f][/name_f][name_f]Hazel[/name_f]
[name_f][/name_f][name_f]Ivy[/name_f]
[name_f][/name_f][name_f]Lydia[/name_f]
[name_f][/name_f][name_f]Mabel[/name_f]
[name_f][/name_f][name_f]Sylvie[/name_f]
Your daughters’ names are just divine, separately and together. It’s too bad you can’t use [name_f]Frances[/name_f], because I think that’s perfect. I think [name_f]Lucy[/name_f] is great, too. The 4-letter thing doesn’t bother me, but I avoided it for you in my suggestions.
[name_f][/name_f]Ideas:
[name_f][/name_f][name_f]Ada[/name_f]
[name_f][/name_f][name_f]Agnes[/name_f]
[name_f][/name_f][name_f]Amy[/name_f]
[name_f][/name_f][name_f]Audrey[/name_f]
[name_f][/name_f][name_f]Betsy[/name_f]
[name_f][/name_f][name_f]Clara[/name_f]
[name_f][/name_f][name_f]Dorothy[/name_f]
[name_f][/name_f][name_f]Edith[/name_f]
[name_f][/name_f][name_f]Ellen[/name_f]
[name_f][/name_f][name_f]Hadley[/name_f]
[name_f][/name_f][name_f]Ida[/name_f]
[name_f][/name_f][name_f]Libby[/name_f]
[name_f][/name_f][name_f]Lindy[/name_f]
[name_f][/name_f][name_f]Nelly[/name_f]
[name_f][/name_f][name_f]Norah[/name_f]
[name_f][/name_f][name_f]Polly[/name_f]
[name_f][/name_f][name_f]Rachael[/name_f]
[name_f][/name_f][name_f]Sally[/name_f]
[name_f][/name_f][name_f]Stella[/name_f]
[name_f][/name_f][name_f]Susan[/name_f]
Thank you, I especially like [name_f]Sadie[/name_f]!
@EloiseT[name_f][/name_f] thank you for the amazing suggestions!
Thank you for the help, I love [name_f]Ruby[/name_f]!
Thank you @kachenka[name_f][/name_f] [name_f][/name_f] I am going to be rereading this over the coming weeks as I continue to find her name. I really appreciate the fresh perspective
Thank you everyone for the suggestions, will be combing over these beautiful names!
I think I would suggest
Eliza
Judith
Harriet
Dorothy
Vivian
Agatha
Beatrix
Celeste
Mabel
Gwenyth
Here’s a list of a few that might work! In my opinion, the best way to connect all three names would be making the third name slightly longer than [name_f]Jane[/name_f] and shorter than [name_f]Marigold[/name_f]. I think this would create a nice balance and would help avoid having one name stand out too much.
[name_f][/name_f]Additionally, I think that having a name that ends in A might help your third daughters name stand out a little more and prevent it from matching too much with either [name_f]Marigold[/name_f] or [name_f]Jane[/name_f]. I also included a few names that are four letter M names because those would visually create a balance between the M in [name_f]Marigold[/name_f] and the length of [name_f]Jane[/name_f].
[name_f][/name_f][name_f]Dorothy[/name_f]
[name_f][/name_f][name_f]Lillian[/name_f]
[name_f][/name_f][name_f]Willow[/name_f]
[name_f][/name_f][name_f]Katherine[/name_f]
[name_f][/name_f][name_f]Helena[/name_f]
[name_f][/name_f][name_f]Caroline[/name_f]
[name_f][/name_f][name_f]Delia[/name_f]
[name_f][/name_f][name_f]Macy[/name_f]
[name_f][/name_f][name_f]Myra[/name_f]
[name_f][/name_f][name_f]Adela[/name_f]
[name_f][/name_f][name_f]Cecilia[/name_f]
[name_f][/name_f][name_f]Cecily[/name_f]