There is a picture of a similarly camouflaged aircraft, with various personnel standing in front of it, also of 7./KG(J) 54, taken at Neuberg-an-der-Donau in March 1945. With the little detail visible in the background of the picture you posted I can only say it might have been taken at the same place and similar time. The weather is the same!
B3 was KG 54's code and had been since the start of the war. You'll have to guess an aircraft letter which following practice at this time should be in the 7 Staffel colour (white). The last letter should have been a black R for the 7th Staffel of the 3rd Gruppe.
IIRC the unit code, B3 was applied in very small letters at this time.
The other aircraft you referred to, B3+BH, was presumably a 1 Staffel aircraft.
Cheers
Steve