How do I complete the task "Bring the Party"?

I'm having trouble completing the Chapter 3 task "Bring the Party".

The in-game text says

Defeat 3 Chitauri with a single attack in Missions 3.4, 3.2 (Hard), or 3.5 (Heroic)

On the game's forums, there is a thread called "Is it a Bug or is it a Feature?" that lists possible bugs. There is a listing for this task, which says:

-Bring the party: Bugged or need rephrasing to "Defeat 3 unharmed Chitauri with a single attack".

So, do I need to kill three Chitauri that haven't lost any health yet?

I tried this on 3.4, but never saw 3 Chitauri at a time. I tried it on 3.2 (Hard), but even my highly upgraded level 30 Iron Man couldn't kill the Chitauri from full health with his missile barrage.

Is it actually possible to find three Chitauri in a single wave on 3.4 normal?

What's the easiest way to do this?