You should use the right-hand lane on a three-lane motorway primarily for overtaking slower vehicles. Once you have safely overtaken, you should return to the middle or left-hand lane. Prolonged use of the right-hand lane without overtaking is discouraged and may be illegal, as it can cause traffic congestion and unsafe driving conditions. Exceptions include heavy traffic, roadworks, or preparing for a specific exit, but generally, the right lane is reserved for overtaking only.