As you are giving help, the following three are all acceptable, only the third is fairly gentle in style.
你得坐公共汽车三站。---> We usually say 你得坐三站公交(公交车): YOu have to take bus and go three stops.
or 你得坐三路公交: YOu’ll have to take bus 3.
你要坐公共汽车三站。.....
你坐公共汽车三站吧。.....