Else you create a timer(multi timer not one shot), show the timer if you want, when the timer ends you spawn a clump of units, or you make a trigger that spawns the units one and one or 3 or 3 (or whatever), in this trigger you add 1 to a variable that you then use it to pick what monster being spawned is.
if this wasent understandable or you need more help feel free to pm me ill answer as fast as i can
