well, if there isn't any other threat to the convoy u could just use this trigger setup:
condition: (not (alive c1)) && (not (alive c2)) && (not (alive c3))
activation: hint "Convoy Destroyed"; t1=true
c1= name of convoy car/convoy ai bla bla, just name what ever it is u want the trigger to wait on.
this trigger will wait for c1, c2 and c3's death or destruction before activating.
on the second trigger do this...
resistance/ present
condition: this && t1
activation:
the trigger will not go off untill the named parts of the convoy is destroyed AND resistance peeps set off the second trigger. U can change "t1" to anything u want, just make sure u change it for both triggers.
hope this helps