How do you set up a three way Facetime call?

FaceTime does not support three way calling.

You don't. Facetime is point to point only.

You would need something like skype (and honestly i'm not sure if they have an app or not) to do a multi point video call

If you want to talk to more than one person over Facetime, have you and everyone else use the Skype app, instead.

Recent versions of Skype will let you video chat with up to 20 Skype users at one time.

Skype for iPad by Skype Communications S.a.r.l

https://appsto.re/us/jqjwA.i