I'm the original poster. Thanks to everyone who has responded.
First let me admit that I misinterpreted OBS above. I thought he was talking about a roomette but was in fact talking about a bedroom (what the web site displays as a "superliner bedroom"). Sorry about that.
Second, if all three of us could eat together in one of the coach dining cars, I think that would be fine with us. So maybe this plan could work.
But I've priced out some other plans. [For (d) through (f) I had to call 800-SKIPI95 because the web site won't let me put 3 passengers in a room.]
a) 2 in a roomette, 1 in coach, 1 automobile (as currently held) $652
b) 2 in a roomette, 1 in coach, 1 automobile (if new reservation were made today) $675
c) 3 in coach, 1 automobile $596
d) 3 in two roomettes, 1 automobile $964
e) 3 in a superliner bedroom, 1 automobile $850
f) 3 in a family bedroom, 1 automobile $1042
g) 3 in a family bedroom, 1 automobile (but travelling one day later) $788
Oddly enough, 3 in a superliner bedroom travelling one day later is $1003, which more expensive than the family bedroom. I don't understand it but that's ok, I don't need to.
I think the promising options are (a), ©, (g), and the unlettered option where one of us flies home and the other two take the roomette ($557+airfare).
Not only does option (e) cost more, but someone said that a superliner bedroom might be too tight for two adults (of appetite) and a teen. I was guessing one in each bed and one in the armchair, but is that not how it works?
I presume that family bedroom would work for us, since the web site pictures it with four beds, but again I'm just guessing. Can anyone confirm? (The web site won't let me book three passengers in a family bedroom. Perhaps they have their reasons.)
From what people are saying, option (a) is risky. Maybe we could all eat together in coach dining, but no guarantees. Maybe the coach passenger could visit the other two in the roomette, if we talk to and/or tip the sleeping car attendant, but no guarantees. Swapping the sleeping location of the coach passenger with one of the roomette passengers seems dubious.