If you're looking for them to complete an Eris bounty, I always find it quicker to do 3 x Fist of Crota missions on Earth. If you just need to kill 3 of them and have no other relevant bounties, just run straight past everything and into the building on the way to Sardon. About halfway through you come to a Blade of Crota. Kill him and leave to orbit. Rinse repeat. There's no competition from other Guardians for the kill, and no waiting on a spawn timer. You kill all 3 in about 5 minutes. If you have bounties like killing Fallen majors (there's one to kill right where you spawn), or melee 30 enemies without dying, killing enemies without taking damage from them, killing 15 Hive with the Sword of Crota, collecting husk shards, generating orbs of light, headshots, killing 100 enemies without dying. All of the above and more can be worked towards or completed entirely with this mission very quickly whilst killing those 3 Blades of Crota without other players competing with you.