If DC3 is a boy we are considering calling him John, to be known as Johnny. This is after DH's elderly & unwell father.
We have friends who have a son called Jonathan known as Jonny.
We see these friends once or twice a year, and will never live in the same place as them. We have one very close mutual friend & some more acquaintance type mutual friends.
I think we can use the name we want to, perhaps giving a polite heads up about it. DH worries. AIBU?