[name_m]Hi[/name_m], welcome to Nameberry 
All 3 names you are considering are wonderful IMO! I love the nature meaning of [name_m]Maxwell[/name_m]. [name_m]Caspar[/name_m] is a favourite and [name_m]Kit[/name_m] is very cool. I love a one syllable name, and gave my son one myself (Beck).
[name_f][/name_f]Here’s some things to consider.
[name_f][/name_f][name_m]Caspar[/name_m] & (lastname)-mas may have repeating sounds, [name_f][/name_f] depending on the pronunciation. [name_f][/name_f] [name_f]Do[/name_f] you like or dislike that, feel neutral about it? Again, depending on PN, do you think that the same ending sound as [name_m]Arthur[/name_m] will lead you to mix up their names in times of sleep deprived/general parenting stress? (I think they are distinct myself and like them so much together 
)
[name_m]Kit[/name_m] and [name_m]Arthur[/name_m] have quite different styles. [name_m]Not[/name_m] like [name_m]Aurelius[/name_m] and [name_m]Gary[/name_m] or anything, but one derives from a nn and the other a classic. Is this something that you care about? On the other hand, they’re both of vintage stock (and I don’t personally think they sit badly as brothers. I like an eclectic sibset in general!) [name_f]Do[/name_f] you have a nn for your [name_m]Arthur[/name_m], BTW? I’m imagining [name_m]Artie[/name_m] & [name_m]Kit[/name_m] now 
[name_m]Maxwell[/name_m]: perhaps a bit of a different style again but everything above I find to be true of [name_m]Arthur[/name_m] & [name_m]Maxwell[/name_m] as [name_m]Arthur[/name_m] & [name_m]Kit[/name_m]. Additionally, how do you the repeating S sounds in [name_m]Maxwell[/name_m] and your last name [name_f][/name_f]- like, dislike, non issue?
[name_f][/name_f]I like [name_m]Caspar[/name_m] so much that at the start of this post I was thinking my personal preference was [name_m]Caspar[/name_m], hands down. But considering [name_m]Kit[/name_m] more carefully, I’ve fallen in love with [name_m]Arthur[/name_m] & [name_m]Kit[/name_m] as brother names! Good luck on finding that name that you both feel is right. Maybe one will click for both of you when you meet him 
Ideas:
[name_f][/name_f][name_m]Beck[/name_m] 
[name_f][/name_f][name_m]Luca[/name_m]
[name_f][/name_f][name_m]Nico[/name_m]
[name_f][/name_f][name_m]Conlan[/name_m]