After two previous Colcester layouts this MK3 one was the second shed based layout and was planned to operate as basically three routes. 
The first route was inside the shed and was designed to run initially from a lower level to the upper level. The second ran simply on one level and served the station of Furman Smithy whilst the third and most ambitious route was planned to run into the garden.
The layout went through many changes of plan due to revised ideas on the track work to make it run better. The gradients were one of the first things to be removed after unsuccessful attempts to get trains longer than four coaches up them failed. The decision was made to make the lower level route an independent DC only one to allow the running of non-DCC chipped stock and test running of new purchases. All the upper level track was made for DCC running.
Continued inprovements to the South, West and Furman Smithy junctions as well as the strorage yard which was put in once the gradients were gone all helped improve the running, however in 2020 it became apparent that there was an issue with the main sundeala boards and the lack of support given to them was causing some notable sagging. This in turn hindered the operation of the trains.
Faced with this and the fact that only about one quarter of the board had scenery on it, it was decided to stop further work and go for a total re-build instead. 
This was originally going to follow the existing plan with a few adjustments made and would be known as MK3a, but it was soon apparent that the opportunity afforded the chance to change the design completely.
This is due to the access to/from the shed and the need to duck under the boards, so the idea was changed to see if it was possible to do a layout in a "U" shape at first.
This however ruled out being able to re-use some key scenic areas from the MK3 layout so a radical new plan was drawn up and this then became a brand new layout dubbed MK4...