3 Adults is doable in a bedroom - from the Blue Book:
If two of the adults don't mind sharing a bed, that's possible in the lower bunk.
If not, you still get more space, more privacy, a private bedroom and free meals for the 3 of you if you book a bedroom.
Remember, the cost of the room(ette) is for the room, not per person. So, you would be paying the lowest railfare for each person plus the cost of the room.
This is a good point as well - when you're booking online, you have to actually click on the room to see what the estimated price will be. For example:
Looking at the Crescent from WAS-ATL tonight you can see that the coach fare is $175 ($350 for 2):
Looking at that, you would assume that adding a roomette means that you would pay $731 ($350+381). However, when you click on the roomette:
What gives? Why is it over $100 cheaper?
When you actually add the seats to your cart, you can see that there is a $126 discount - that's the difference between the lowest railfare and the current railfare. In this case, we can see that the lowest railfare is $112 per person, a discount from the current railfare of $175 per person: