Simply kill 3 people with grenades or you can use a bazooka or a panzerschreck which will still count towards "Bomerman."
I managed to unlock this trophy by using a bazooka and killing 3 people with it in a match.. It still counts.. So I can confirm that which it might be easier but Bazooka's and panzerschreck's are quite rare to see in this game so please be mindful of that or you can just lob grenades at a team or in a high traffic area and hope that you get a kill or a couple of kills.