These three dry docks were part of the Mount Stuart Shipbuilding Yard (NPRN 300173) under the control of the Mount Stuart Dry Dock and Engineering Company. The docks underwent several alterations over the years.

Dock number 1 was originally 440ft (134.112m) in length and 52ft and 3in (15.9258m) in width, but was later altered to 420ft (128.016m) in length and 52ft (15.8496m) in width before finally being enlarged again to 430ft (131.064m) by 52ft 9in (16.0782m).

Dock number 2 was originally 420ft (128.016m) by 52ft (15.8496m), then shortened to 400ft (121.92m) before being enlarged to 495ft (150.876m) by 63ft (19.2024m).

Dock number three was originally 550ft (167.64m) by 66ft (20.1168m), before being extended to 565ft (172.212m) and then shortened to 543ft (165.506m).
When the hitherto tidal inner harbour of Cardiff Bay became a permanent freshwater lake in 2001, the docks were filled with water.

(Sources: Allen, `Engineering and Shipbuilding? in Ballinger, Cardiff: An Illustrated Handbook (Cardiff: 1896), pp. 114-5; Hutton, An Illustrated History of Cardiff Docks, vol 2 (Great Addington: 2008), p. 136)
A.N. Coward, RCAHMW, 03.08.2018