Play PVP Matches which are called the Crucible in Destiny.
Every Time you kill enemy Players (Kill +100) or even assist in a Kill by your Teammate (Assist +50) it will count towards your Total.
In the Heat of the Action (for me at least) it's hard to tell which of the three Enemy Types (Hunter, Warlock, Titan) you've killed.
So maybe it'll help you to go to the Bounty Board and get some of the "kill 25 hunters" Bounties, which will be available again, a short time after you turned them in.
So you won't need a total if 300 kills to get yourself those 3 trophies, cos assists count as kill as well.
How fast you'll get those trophies depends on your level of skill and luck (and your weaponry) but even for a mediocre PvP player as me, it doesn't take long.
Good Hunt Guardians