I feed my ducks and geese Mazuri Waterfowl (starter, maintenance and breeder when they're laying). I start to mix in maintenance with the starter as they get older.
I recently started mixing into my new ducklings' and geese' feed a locally milled feed that is specifically for ducks and geese, too. There have been times when my feed store was out of Mazuri and my older ducks turn their bills up at anything but the Mazuri. It's nice to have a back up. I'll probably continue to mix both together so they will eat both in case there are times when I can't get one or the other.
3 months is well past. We switch when we start to see wing feathers around here. Protein can help create angel wing, so we reduce at first signs of feathers starting. No need to rush things along.
Oh and Christine, thank you. Had been asked a few times what we feed, it isn't as day and night as it reads (some won't eat the black oil sunflower seeds lol) but its the base recipe of feed and we alter when needed.