In-order to allow running at 125 mph on Britain's railways, new rolling stock was needed by British Railways, and a new Mark entered service in 1975. Significant improvements over the Mk2 included new secondary air suspension between the bogies and the coach body as well as aerodynamic skirting on the underframe. Mk3 Coaches are 75ft (23m) long enabling far greater capacity than older coaches. Mk3 coaches also incorporate disk breaks and wheel slip protection enabling faster deceleration.

The first Mk3 Coaches to be delivered were used as part of the HST prototype along with the two Class 41 diesel power cars in 1972. Mk3 coaches entered service in 1975 along with the Class 43 forming the iconic InterCity 125 trainset.

After the HST Mk3 Coach variant was introduced, further Mk3 Coaches were introduced to the West Coast Mainline for use as part of locomotive hauled trains. Mk3s remain in service as part of HSTs with East Midlands Railway among other companies

The British Rail Mk3 Coach was developed primarily for the Class 43 HST, but incorporated design features to enable it to be hauled by conventional locomotives. The first coach entered service in 1975 and the last were made in 1988. Most Mk3 Coaches built are still in service today, including a part of the Royal Train.