Kinda depends. Some hotels (usually the nicer ones) are pretty particular about the number of people staying in the rooms. Among other reasons, it relates to fire codes. For instance, we are a family of 6, but our kids are all 10 or under. We can't usually book a double room (ie with two double beds, or one double and a pull out sofa) even though our kids have no problem sharing a large bed - or sleeping on the floor, because they have a limit of 4 people in that room. So a family with two kids could stay, but three kids, no way. 4 kids? Forget about it.
We usually end up booking two rooms, and splitting the kids between us, or, if we luck out with connecting rooms, throw all the kids in one and have one to ourselves. With 6 grown ups, you can easily get a room big enough for three/room, but it will probably be a more expensive room. If you try to cheat the system and sneak the other two guys in to a single room, I don't imagine they'll let you stay more than one night. Besides, in a lot of cases, a single room has - get this - a single bed. I mean a twin bed. So the other two guys will be sleeping on the floor. Maybe at the Marriott you'll get a better deal, but we stay at cheap hotels where 1 bed is big enough (barely) for 1 person. You might consider calling a hotel in a different city (Lyons or Strasbourg for example) and asking them about their policies - be honest, ask what kind of room you need for three adults. Take that info and work out how much more you'd have to pay for that room than the room you're planning to book - if it isn't much, I'd just have everyone chip in a couple bucks and upgrade. Then you end up with enough beds, towels and what not, and don't risk getting kicked out when they catch you.
If you're traveling to other places, look into the Etap and Formule 1 chains - these are where we normally stay. They have bunk beds - a double/twin combination and a private bathroom (in the Etaps) and not much else, but in Paris we stayed in one for 43 Euro/night, which is roughly 15 Euro/person, so they compare favourably to hostels, a lot of the time. The Formule 1's are cheaper, usually around 30 Euro/night, though they have shared bathrooms. Still, 10 Euro/night isn't too shabby. Our Etaps in Vienna and Salzburg were about 35 Euro/night for three people/room.