I was wondering about this and a bit of not particularly good googling hasn't helped.... what happened to the Mk 3 the 390 displaced - there were quite a few from what so can see.. certainly more than 70?
Some of the numbers below might be slightly out of date:
Mk. 3a RFM: 58 Built/Converted (10200 – 260, with some gaps), 36 mainline registered
ACTIVE: 4 with GWR, 2 with Arriva Trains Wales, 2 in Pretendolino rake(s), 7 converted to RSM for Greater Anglia, 4 converted to power door fitted TFK for Chiltern Railways, 1 in DB Cargo company train, 2 used as instruction coaches with Scotrail at Yoker.
STORED: 12 in store with DB Regio, 1 in store with Porterbrook, 1 in store with West Coast Railways.
Also 1 in preservation with the 125 Group (10206), 1 in preservation at the Mid-Norfolk Railway (10213), 1 in use as holiday accommodation at Hawsker (10218) and 1 at the fire training college at Moreton-in-Marsh (10231) (Grand Total = 40 vehicles).
Mk. 3a FO: 60 Built (11004 – 063), 18 converted to Mk. 3a RFM, 4 converted to TSO, 19 mainline registered as loco hauled stock
ACTIVE: 5 converted to power door fitted TSOs for Chiltern Railways, 3 converted to TSOs for ATW, 3 in Pretendolino rake, 2 with Chiltern Railways, 1 in DB Cargo company train.
STORED: 3 in store with DB Regio, 1 in store with DRS, 1 in store with Porterbrook.
Additionally, 3 have been converted to HST FO vehicles for Crosscountry and 6 into HST FO vehicles for Grand Central (Grand Total = 28 vehicles).
Mk. 3b FO: 38 Built (11064 – 101), 37 mainline registered
ACTIVE: 28 with Greater Anglia, 7 converted to TSOs for ATW.
STORED: 2 in store with DB Regio.
Also 1 in preservation with the 125 Group (11074) (Grand Total = 38 vehicles).
Mk. 3a TSO: 165 Built (12004 – 168), 4 subsequently converted from Mk. 3a FO (12169 – 172), 126 mainline registered as loco hauled stock
ACTIVE: 77 with Greater Anglia, 16 converted to power door fitted TSOs for Chiltern Railways, 6 converted to RSM for Greater Anglia, 5 with Chiltern Railways, 5 in Pretendolino rake, 3 with GWR.
STORED: 9 in store with Porterbrook, 2 in store with DRS, 3 in store with DB Regio.
Additionally, 15 have been converted into HST TSO vehicles for (primarily) Crosscountry and GWR and 9 into HST TSO vehicles for Grand Central. A further 5 have been converted into HST TCK vehicles for Crosscountry.
Also 4 in preservation with the 125 Group (12065/087/092/134) (Grand Total = 159 vehicles).
Mk. 3b BFO: 3 Built (17173 – 175), 3 mainline registered
ACTIVE: 3 with First Great Western.
In addition, 3 Mk. 3a FOs and 5 Mk. 3a TSOs were scrapped as a result of accident damage during the 1980s (Bushey (1980), Polmont (1984) and Colwich (1986), primarily). 12140 also subsequently scrapped following withdrawal from service with Virgin West Coast due to being fitted with non-standard T4 bogies.