Hmmm that’s a tricky one!
I have a niece called Belle and it’s a charming and also (perhaps surprisingly given princess association) easy-breezy name in real life. But it’s normal to fall out of love with a name! The hard part is in figuring out if it’s late stage preggo nerves/hormones or a genuine realisation 
.
Meanwhile I have a friend with a daughter called River, which is Cali/beachy cool and an immediately likeable name.
River doesn’t work quite as well with James and Rose though. I’m someone who says always choose name for its own sake first (as child mostly their own person) and for coherence of sibset second, but I also think it’s good for sibling relations and kids’ identity (re who was favoured etc) if the names are broadly equitable - eg and I’m saying this off cuff so maybe not the best eg, James and Rose could feel like River got the the cool, “special” name, or River could wonder why she got the more unisex, slightly more “out there” name and not a classic. James and Rose are a little more versatile - for all ages & contexts.
I also think I’d try for a non R name, unless River or another R name really is your all-time love!
I wonder if there’s a Third Way, something peppy but still fairly classic and/or vintage, eg James, Rose &
Thea
Eve (one of the loveliest)
Piper
Daphne
Zoe
Laurel
Sienna
Ingrid “Indie”
Willa
Ivy
Cora
Oona
Phoebe
Meadow (like River but more Fem and tailored) Avalon (as above)
Winter (as above)
Olive (as above)
I know it’s the middle, but I’m genuinely vibing with Marie/Mary/Mara/Maria as firsts these days. Something so “clean”, spunky. and essentially pretty about them. James, Rose & Marie or one of the others on the list would be fab! Marie River or Mary River nn Mimi/Miri? Also means following the more classic first + more fun/creative second. Or as other have said, maybe mixing it in: Belle River / Belle River Marie/ Belle Mary River?
Marie River or Mary River would be my pick!
Hope this helps?