In the past, we have stayed with our young-adult daughter at what is now called Coronari Palace. I love the location, so close to Piazza Navona and close walking distance to Camp de'Fiori and the Pantheon. We're big walkers, and easily also walk from there to the Vatican, the Forum, etc. At that time, her bed was in an entry alcove that had a door between her and the bed that me and my spouse slept in.
Note that per Google maps, it's a 4.8 km walk from Termini train station to the middle of Vatican City (also, the entrance to the Vatican Museums is on the north side of Vatican, though the entrance to St. Peter's is on the east side). You may want your companions to decide what they want to be close to, if those distances aren't exactly what you have in mind.
I think that to confirm what kind of bed the third bed would be, you would want to look at each room description and look at photos. And if still in doubt, send an email to the hotel.