I keep on trying to merge 3 rooms but 1 makes a different one! Do I have to wait later in the game?
The rooms must be of the same type, upgraded to the same level and placed next to each other horizontally (with no gaps) in order to merge into each other. It is possible to build rooms of the same type but different upgrade levels next to each other and these will subsequently merge when they're upgraded to the same level.
I would also add that the rooms must be adjacent horizontally (not vertically), as that particular issue tripped me up the first find I tried to merge rooms in the game.