Say there are two or more creatures with legendary actions in an encounter, can only one legendary action be performed after a creature's turn, or can multiple be done?
I don't see why they can't all use legendary actions. In that instance, resolve them in order of the monsters initiative.
so if a creature goes, then say 3 legendary actions can be taken by 3 monsters all at once?
If the encounter has 3 legendary creatures, yes. Such an encounter should be pretty unlikely though.