You can indeed take down all three Eldrazi this turn, although you’ll need to maneuver around their abilities. For the record, Emrakul flies and has protection from colored spells, and Ulamog has indestructible. They don’t compare to a well-timed Orzhov Euthanist, however…
- Tap five swamps to add
(two used as generic mana) and two mountains to add 
(two used as generic mana) to your mana pool. - Tap Vithian Stinger to deal 1 damage to Emrakul. (This bypasses Emrakul’s protection ability since the source is a creature in play, not a spell.)
- Spend
to cast Orzhov Euthanist. When Orzhov Euthanist enters the battlefield, choose to destroy Emrakul. (Again, this bypasses Emrakul’s protection ability because, by the time the Euthanist’s ability triggers, it is a creature in play, not a spell.) - Spend to cast Tremor. This deals 1 damage to Festering Goblin, Vithian Stinger, Orzhove Euthanist, the Eldrazi Spawn token, Fauna Shaman, Ulamog, and Kozilek. Festering Goblin, Vithian Stinger, and the Eldrazi Spawn token die.
- When Festering Goblin dies, its ability triggers. Choose to give Orzhov Euthanist -1/-1 until end of turn.
- Orzhov Euthanist is now a 1/1 creature with 1 damage on it, and dies.
- Orzhov Euthanist’s Haunt ability triggers. Choose to exile the Euthanist haunting your Death’s-Head Buzzard.
- Spend
to cast Funeral March on Kozilek. - Spend to unearth Vithian Stinger. Vithian Stinger returns to the battlefield with haste, and will be exiled at the beginning of your next end step.
- Tap Vithian Stinger to deal 1 damage to your Death’s-Head Buzzard. Death’s-Head Buzzard dies.
- When Death’s-Head Buzzard dies, two abilities trigger: the Buzzard’s “Give all creatures -1/-1 until end of turn” ability, and your Orzhov Euthanist’s ability. Choose to put the Euthanist’s ability on the stack, followed by the Buzzard’s (so that the Buzzard’s ability resolves first).
- The Buzzard’s ability resolves, giving all creatures -1/-1 until end of turn. Fauna Shaman is now a 1/1 creature with 1 damage on it, and dies.
- The Euthanist’s ability resolves. Choose to destroy Kozilek.
- Funeral March’s ability triggers, forcing Bosko to sacrifice a creature. Because Bosko now controls no creatures other than Ulamog, Bosko is forced to sacrifice Ulamog to the Funeral March.