Handwritten caption on reverse: '3 FG, 12 AF.'
It is more likely the aircraft was assigned to the 3rd Photo Reconnaissance Group, since there was no "3 FG (Fighter Group)." It is difficult to tell the full aircraft serial, but the presence of the 'last three' (194) on the vertical fin is additional evidence that it is likely a recon unit, since this was a highly common ID method; none of the 12th AF fighter units claimed similar markings.
IWM catalogue record
This contains information written on the back of the original print and some of it may be inaccurate.
Ground personnel of the 3rd Fighter Group, 12th Air Force inspect a P-38 Lightning that has crash landed in a ditch. Handwritten caption on reverse: '3 FG, 12 AF.'
The 3rd Photographic Group were stationed only briefly in Britain and flew no missions from either Membury or Steeple Morden. They were reassigned to the Twelfth Air Force and undertook photographic reconnaissance in support of operations in the...