Your girls have stunning names!
Jack Wells has some of that spark and friendliness of Halle and Willa, that quirky-nickname-y-playful vibe, while still feeling versatile and grounded.
For me though, I do feel like Jack doesn’t quite have the same shine as Willa and Halle - maybe because I’ve met so many Jacks/Jacksons/Jaxons etc - and it does remind me a bit of the brand, Jack Wills. That’s not to say in anyway that it’s a bad choice - and it’s familiarity and versatility might be part of the appeal to you 
A couple of other ideas:
Halle, Willa, and…
Kit
Jem
August / Gus
Mack
Beckett
Beck
Casper
Felix
Finley
Flynn
Otto
Ruben
Bowen
Royce