QI have 3 followers on my field, and I attack an enemy leader with this follower. If an opponent destroys one of my other followers with a Quick ability, do I still give this follower +1 attack with its Strike ability?

AYes. During combat, your Strike abilities are resolved before opponents' Quick abilities; the active player's pending abilities are resolved before non-active players are able to respond.

In this case, even if a Quick ability would cause the number of followers on your field to drop below 3, this follower's Strike ability will have already been resolved.