I understand that the 156s were 3 car at somepoint in late 80s/early 90s, mainly working between Inverness and Aberdeen in Scotland.
Also the West Highland Line.
Inverness - Aberdeen was operated with 3-car sets from May 1989 to May 1990, 156495 & 496 were split with vehicles added to 156485 and 492-494 to make up 3-car sets. 458 stayed as 2-car, nominally the maintenance spare.
Delays in the 158 programme meant the sets were needed for Express duties out of Haymarket Depot, standing in for what should have been 158s by then. Sets were all reformed to 2-car, with Glasgow to Aberdeen usually one set (now hourly) and Glasgow to Edinburgh two or three sets (still half-hourly).
Aberdeen to Inverness reverted to loco-hauled and Edinburgh to Aberdeen ScotRail services remained loco-hauled and the planned frequency increase to hourly was delayed.
The other Scottish 3-car sets were run on the West Highland, in the summers of 1989 and 1992. 6-car was the maximum most stations could take hence 3 to Oban and 3 to Mallaig, as the new timetable called for portion working to Crianlarich, where previously the Oban and Fort William/Mallaig trains ran separately.
Outside Scotland, 3-car sets also operated on the Settle-Carlisle, as certain platforms there could only take 3-cars pending extension. 3-cars only ran for the winter of 1990-91, extensions to allow 4-car workings were completed for the summer 1991 timetable.
More details can be found on this website:
http://members.madasafish.com/~dysgraphyk/156/class156_misformed.htm
Misformed Sets
The planned formation of class 156 units is DMS(A)+DMS(B) 2-car sets with the last 3 digits of each vehicle matching the set number. This ensures that both vehicles receive scheduled maintenance at the same time.
However if one vehicle in a set needs to be stopped for unplanned repairs, and there is a shortage of serviceable units, temporary formations may need to be created.
In the early years an number of planned three-car formations were also created for use on specific routes.
