Narratively they could be hitting more than one target, mechanically the Wound Threshold of the minion group is the same.
A group of 3 minions with 5 Wound Threshold each would be a total of 15 (5 + 5 + 5).
Here's the part that is confusing to many people:
When the current wounds exceed the threshold of an individual minion, a minion is destroyed (which minion is dependent on the narrative)
So the first minion would be killed at 6/15 (because 6 wounds exceed the 5 wound threshold), and another would be killed at 11/15 and the last at 16/15.
Essentially the damage of killing blow of one minion will spill over into the next minion, so narratively there could be more than one target. This is okay, because the job of a minion is to give the PCs a challenge but to die off quickly.
Edited by kaosoe, 22 July 2014 - 11:14 AM.