When we used to do this (usually 5 people in a room with two doubles), the deal we made was that Floor Guy slept for free. (This happened mostly in college or soon thereafter.) We always had one guy willing to be Floor Guy.

If you actually want three twin beds, you'd better book them properly for London or Paris. Unless you want an unwitting Floor Guy.

I don't think hotels are just trying to rip you off by jacking the rates up on rooms that can comfortably sleep 4 people. The rooms are, in many cases, simply smaller and properly fit for the number of occupants listed.