Bachmann have released 3-car 108s at various times - I bought one when they were in the Hatton's or Rails' bargains (can't remember which of those shops it was!). Beware of using BR mark 1 coach sides as they are usually the wrong profile and have wrongly sized windows for the DMUs - the Cravens units were the only ones to use standard Mark 1 windows, classes 105, 106, 112 and 113, although only a relative few were 3-car units. DoubleDeckInterurban is absolutely correct that there were many hybrid units run. One book I have even has a class 104 DMBS heading a class 124 TransPennine unit in blue and grey - something I would not have expected if I hadn't seen the picture. Class 101 centre cars were probably the most frequently seen substitutions in other unit classes; certainly they have run within class 104, 108, 110 and 120 units at various times, and also within really mixed units.