A super level attack is performed by pressing the R shoulder button. You can have 3 different levels of super level, your level is indicated by the blue bar underneath your characters portrait.
This trophy seems to be bugged for me and came down to luck more than anything. I got the 3 kills with Kratos, PaPa and big daddy but it didn't unlock. Finally I used toro who has a good horizontal charge as his level 1 super. Group your opponents up, jump to one side then launch your super when at level 1.
Hopefully that saves anyone else the frustration :)