You need to use the count command. This is easiest to do if you have each of those AI units in one group.
In the initialization field of the leader of the group, put the following code:
squadUnits = units (group this)
This creates an array, squadUnits, that contains a list of every unit in that group.
Then, put the following code in the condition field of a trigger:
"not alive _x" count squadUnits > 0
This checks to see if there is at least 1 dead unit in that group, and if so, the condition is met.
If you do not have all of the AI in one group, you can still use this code, but you must instead manually create an array with a list of the units.
E.g.:
squadUnits = [guy1,guy2,guy3]
This code can be placed in the initialization field of any one of those units, or in your init.sqs for the mission. The trigger's condition would remain the same.