You can get this trophy much easier than it sounds by incorporating Death Blossom into your three kills. What this means is that if you damage an enemy with your shotguns before you take them out with Death Blossom it will count toward Waste Not, Want Not. I can confirm this as this was how I got the trophy. What I recommend is not even trying for the trophy until you have your ultimate ready. Once it is ready to go, seek out an enemy support preferably Zenyatta. Any low HP hero will do I simply prefer Zen since he is easy to take out if you get the drop on him. Once you have your first kill seek out two more enemies who are close enough together to be killed by Death Blossom. You will have to use your personal judgement on who you use your ultimate on and whether or not you think they will die from it or kill you while you attack. Good examples would be D.Va and Mei. Even if characters like this are weak or unsuspecting, they have abilities that can ruin everything like D.Va's Defense Matrix or Mei's Ice Wall. Sneak up on them and make sure you land at least one shot into each of them with your shotguns then active your ultimate and kill both of them. If all goes well you should get the trophy once the second enemy(third enemy in total) dies from Death Blossom. Good luck and remember not to reload in between kills.