Here is how to find the keys and craft the infernal machine!

Key 1 is Found on Goat Mutant Key Warden in the Fields of Misery (Act 1)
Key 2 is Found on Dune Derzvish Key Warden in Dahlgur Oasis (Act 2)
Key 3 is Found on Morlu Spell caster Key Warden in Stone fort (Act 3)
Infernal Machine Plan – Found on Terror Demon Key Warden in Silver Spire level 1 (Act 4)

    All of the Key Wardens will always spawn in every Inferno game.
    You must have a 5 stack of Nephalem Valor for the drops to even have a chance of dropping.
    If you do it on monster level 10 with your 5 stacks, you are guaranteed the drop.
   It does not drop every time, and if it doesn’t drop, you need to restart the quest.

Here is how to use the Infernal Machine
Once you’ve created the machine, you must activate the portal to enter the three realms that contain the Uber Bosses.

The machine needs to be activated in the Heretic’s Abode found in New Tristram.
In order to enter the Heretic’s Abode, you need to be on the last quest in Act 1.
The portal will open up one of the three locations randomly.
The Realm of Discord – King Leoric and Mahgda.
The Realm of Turmoil – Siege Breaker and Zoltun Kulle.
The Realm of Chaos – Ghom and Raknoth.
The machine only works to open a portal one time.
A new machine will need to be crafted each time to open another portal.
The portal remains open until the bosses are defeated or you leave game.
The Vengeful Eye, Devil’s Fang, and Writhing Spine, needed to craft the Hellfire Rings, are not guaranteed drops.

Here is a video, not created by myself, that shows visually how each step goes to craft the Infernal machine – Enjoy.