I'd say the one in back is a Rouen, and the other two possibly Buff Orpington? too early to tell the sex by a picture..when they start to quack the females have a loud quack and the males is much softer.
The black & yellow duck is a runner/welsh mix (or so I was told at the swap where we got him/her). The bigger yellow one is getting really really dark on it's back. It doesn't really matter what breed they are....I'm just curious.
The yellowish one with the dark bill is a welsh harlequin
The other is harder to tell. I would say Buff. but I do not think they are the same age. The harlequin looks to be 1-2 weeks older. You will know for sure when they start to feather. If one gets feathers while the other doesn't, then there is clearly an age difference