You choose the target creatures to support when Jubilant Mascot's triggered ability is put onto the stack, but you don't choose whether to pay {3}{W} until the ability resolves.

You can't put more than one +1/+1 counter on any one target using the support action.

Support can target a creature another player controls.

If a spell with support has other abilities that target creatures, those abilities and the support ability can target the same creature.

If some, but not all, targets for a spell become illegal, the remaining targets are affected as appropriate. If all of a spell's targets become illegal, that spell doesn't resolve.