George raised a good question about a pair raising three youngsters in the nest.
It got me thinking.
I have seen pigeons raising three youngsters, either by accident or design. My mate David has/had a hen that laid 3 eggs in every round, and they raised them. I have a hen off her, and she did it once.
What is not usual is that once, when out of options, I put a pair of small babies that had been abandoned under a pair of old stock pigeons. That old hen had not laid after weeks and weeks in the nest box. I hoped they would at least keep the babies warm. Instead, they adopted those little ones instantly and raised them. Amazing!
I never hesitate to move eggs and babies around when needed.
Fostering.
I think once over the milk stage you can quite easily move youngsters around. I often move youngsters around to make pairs of similar size if one youngster is falling a bit behind the other. Some birds will feed anything. I had an old cock last year that when sitting his own eggs kept going down into the nest box below him to feed the youngsters in there.
Back just enjoying club racing for the time being.
I also quite often move babies around, I have sadly had it go badly wrong a couple of times though and found the moved baby beaten to death
you certainly need to get your timing right and know your pigeons as some will take anything at any time whereas some just have no parental instincts at all.
On a slightly different note but still following on from George's post, I currently have two pairs of yearlings sharing one nest bowl 
Both hens have laid so they are sitting 4 eggs, they seem to take it turns sitting them on a first come first served basis 

