have all the templates made before hand next add a global/instance variable that titled something like stage decider
then add a event that goes every x seconds (the time it takes to pass a template) or have something at the end of each template that will trigger an event eg(on P1 overlap stage trigger run) and have it add a random number to the variable use the round random for that ( you can find more info about that on this site ) but it should look something like this round(random(1,10)) then simply add conditions testing for the various numbers and have it spawn the required template
eg
If stage decider=1|Spawn Temp1
|Set stage decider to 0
If stage decider=2|Spawn Temp2
|Set stage decider to 0
If stage decider=3|Spawn Temp3
|Set stage decider to 0
remember to clear the value after every outcome
If you need anymore help just ask