Officially there is no room that can hold 3 adults. The Bedroom has more than enough room during the daytime hours for 4, perhaps even 5 adults however. The problem is at night, where there are only two beds and three people. I would suggest calling back and trying again, as it's not uncommon to get different answer from different agents.

You could also try calling and asking specifically for a Supervisor, since they are probably the only ones that can override the system anyhow. Still no promises, but you can try that. Explain to them that you understand that there are only two beds and that you are willing to share the lower bed between two people and that the third will go into the upper bunk.

The issue here is also partly that the crews aren't very good about verifying that people entering the dining car are actually entitled to meals. Officially you only get two meals per room for each meal period. But many crews don't stop to count if three people show up from the same room, and therefore don't force one of the party to pay for their meal.

You could also consider getting two roomettes, as there are times depending on just how sold out the train might be, where two roomettes would cost less than a single bedroom.

Do not however try booking two adults and one child into the room, unless one of your party can easily pass for a child. If you do that, the conductor may well deny the so called "child' passage as you did not pay the proper railfare for an adult. You paid for a child. Not saying that it will happen, but the conductor could toss that person off the train and even call the police and have them arrested for fraud.

Best case scenario would be that he/she would demand payment for the adult fare. They could also demand payment and relagate the third person to coach, assuming that there is room.