Danger Scavenger

It pretty much goes without saying that you'll earn this naturally during normal play, however if you can't be bothered or need to snipe an achievement for a contest or something, the training dummies at the beginning where you select a gun do count. I haven't done any analysis of which character or gun to pick as I was already close and just mopped it up this way, but Captain J had a shotgun which could hit all 3.