Many of the locations for rooms are puzzling - a lot of the 2-room cabins are squeezed between other "working" parts of the ship, not in a dedicated accommodation section as you might expect (and indeed directed to by the signs, such as the oft seen "OFFICER'S QUARTERS")
As for Garrovick it is still a puzzle, as he seems to have just as much space as Kirk (the second area of the cabin is visible in one scene). So, what makes it third class?
Although he does hold the rank of Ensign, he seems to be a high level security officer in the episode (and Rand got her own cabin). Maybe that is why he gets a room to himself?