Indeed they did, during the summers of 1989 and 1992 a number of Class 156s were reformed into 3 car units.
The 3 car units during Summer 1989 would have been
156.445 formed of 52445 + 57499 + 57445
156.447 formed of 52447 + 52500 + 57447
156.449 formed of 52449 + 52499 + 57449
156.450 formed of 52450 + 57500 + 57450
156.453 formed of 52453 + 57435 + 57453
156.456 formed of 52456 + 57435 + 57456
This was made possible by splitting units 156499/500 when they were delivered to Haymarket in May 1989 and added as the centre car to four of the above, when 156435 was delivered to Haymarket from Corkerhill in July 1989 this enabled the last 2 units above to be made into 3 car sets.
They were however reformed into their original 2 car formations in September 1989.
As to Summer 1992, the entire West Highland fleet was transferred over to Corkerhill in May 1992 and between July to September 1992 again BR formed 3 car sets by splitting 156436/492/496.
In May 1992 the West Highland fleet was transferred to Corkerhill depot in Glasgow.
From July to September of that year 3-car sets were formed once more, by splitting units 156436/492/496.
156.445 formed of 52445 + 52496 + 57445
156.449 formed of 52449 + 57492 + 57449
156.450 formed of 52450 + 52492 + 57450
156.453 formed of 52453 + 57496 + 57453
156.465 formed of 52465 + 52436 + 57465
156.495 formed of 52495 + 57436 + 57495
More information can be found
here.