I have 2 girls & 1 boy.
My boy Alfie is 6.....then our 1st girl Jo Jo who is nearly 5 yrs old came along....now we have Nina too, she is 2.8 yrs old.
Initially Alfie couldn't bear to even look at Jo Jo, and for many months i felt terribly guilty for upsetting him
over time though he grew to love her & they absolutely adore each other now.
Nina gets on really well with her mum (Jo Jo) but sometimes she can irritate/wind up Alfie
I would say overall though, that they do all live quite happily together. One important part of their live's though is to try to have some 'one to one' time with each of them, this could just be a walk/activity/training because they enjoy this and it creates a stronger individual bond iyswim.
Personally, i would probably have another girl if i were in your situation.
There are never any guarentees though that all your dogs will bond/like each other etc...us humans are the one's that create the situation with multi dog households, so we have to try to make sure that any potential probs within the 'pack' is sorted out to enable them to live as happily as possible together.