set the triggers to repeating. set the 'timeout' part of the trigger to min = 1, max = 5, mid = 3. to randomise where the explosions land within the trigger, name the trigger. then in the 'on activation field' put
expl = "shell125" camcreate [(getpos trigger_name select 0) + (random 30*sin(random 360)), [(getpos trigger_name select 1) + (random 30*sin(random 360))]
a little untidy - it should really be done in a script - but it should work.