Railfan,
No matter what you will not be able to price this out online, the online system will not allow 3 people into any room, other than a family room.
Now that said, at one time Amtrak did allow 3 people in a roomette, provided that one of them was a small child. That usually meant under 5. I'm not sure if that policy still stands, I've heard conflicting reports of late. I do know that it is possible to put 3 people into a bedroom, the larger of the rooms. But the only way to accomplish any of this is to call or visit a station.
After too many people booking three into a sleeping compartment and then complaining about the lack of space, Amtrak no longer allows online bookings. Any time an agent agrees to override the normal rule of 2 in a room, he/she will give you fair warning that Amtrak does not recommend this, as it can be tight.
So again, you should definately be able to get three into a bedroom, may be able to get three into a roomette (but I won't promise that), and you'll have to call or visit a station to accomplish either.