What does double-check mean in four-player chess on chess.com? The rule says:
Delivering a double-check is worth +5, and triple-checks are worth +15

Is a double check when I am 


checking two players at one move
checking one player with a normal double check
checking a player that is already under check


Counting that there is a "triple check" either the rule one or three should be true because you can not give a triple check to one player by only your move. However, I am not sure if there are points for the normal double-check.
Double Check in 4 man chess basically means  checking  two kings in one move.
Same goes for triple check.

PS. It's not worth the points so don't go for it unless you win because of it.