Three-camera shows are faster to shoot which is why they’re used on sit-coms. The biggest disadvantage is that the cameras don’t move much, which is why the shots look exactly the same show after show. (The shot of the living room in the first episode of Friends was exactly the same as the shot of the living room in the second episode, and the 100th, and the last.)
One-camera shows are harder and take more time but are much more flexible because they use cameras which move and they use steadicams. “One” camera is a little misleading, because while most scenes are shot by a single camera, there’s a good chance there’s a 2d unit guy shooting other scenes down the hall at the same time.
Single-camera is used in most dramas as well as certain sit-coms that have a distinctive look (Arrested Development and Scrubs come to mind). Single camera can deliver a lot more emotion by being in exactly the right place to catch the actor’s glance (and by not looking so staged as 3-camera shows do), but there are downsides – you have to shoot the most basic dialogue scenes a minimum of three times (over A’s shoulder, over B’s shoulder, and the master or two-shot which has both) so that you can switch to speaking shots and reaction shots and two-shots to capture the mood and the best performance. This can be draining to actors in emotional scenes.
Of course, you can do a million other things with one camera – tracking shots, long-ass takes that follow characters from room to room, weird blocking, shooting up and down to make things look a different size, cinema verite, the room spinning disorientation shot, etc., etc., as well as shooting on location. You can’t do any of that with the three camera set-up, but you take a lot of time and money lighting and blocking all those neat shots – after half a dozen episodes of a 3-camera show your actors know exactly where to stand, but on a single-camera show the actors have to learn it for every scene because the camera position is always different, as is the lighting.
You can also shoot three-camera in front of an audience, which you wouldn’t do with one-camera because it constrains the ability to move that is the whole point of using a single camera – contrast the views you saw of Monica and Rachel’s living room (basically just two) versus the views you saw of Buffy’s (360 degrees). There was an audience on one wall of M&R’s apartment, but on the 4th wall of Buffy’s room there was just a wall.
If you can, take a look at Scrubs’ recent episode “My Life in Four Cameras.” (I don’t know why 4 instead of 3.) The first half of the ep is single camera as the show usually is, then they switch to multiple-camera sit-com format in an extended fantasy sequence. The fantasy bit makes a lot of jokes about how unsophisticated sitcoms are, but as far as the look of a 3-camera show, they get it exactly right (not surprising, since I’m sure all the crew has worked on 3-cam shows). The difference is very clear. 1 and 3 camera styles are completely different and once you understand the difference it’s obvious which is which within literally seconds.
–Cliffy