@Nick P:
On the one hand, completely untrue. There's nothing anywhere saying sitcommission entries have to be in three acts.
You may have heard someone talking about a 'three act structure', which is a classic way of looking at story structure. A story can have a 'three act structure' but still be set in one room in one continuous take (see the Psychoville 'Rope' episode for example. Or indeed, 'Rope').
On the other hand - I personally wouldn't recommend one 15 minute scene in one room as it's likely to seem a bit static and probably 'talky'. If the writing is brilliant, you may overcome this, but you're not making it easy for yourself. Having said that, IIRC "one room, one scene" applied to last year's finalists 'Eye Captain', 'The Box' and 'Up on Bricks'.