Well assuming the three targets are foot units (not vehicles) called unit1, unit2 and unit3 set the condition field of the trigger to
not (alive unit1) and not (alive unit2) and not (alive unit3)
(or:
{alive _x} count [unit1,unit2,unit3] < 1
)
If they are vehicles try the following in the trigger's condition field:
{(canFire _x) or (canMove _x)} count [unit1,unit2,unit3] < 1