The T meant Technician, three different grades: T3, T4 and T5, they were NCO ranks. The technical grades held the same pay grade as Staff Sergeant, Sergeant and Corporal respectively but from what I can find out ranked just below those ranks with regard to chain of command.

The technicians generally had a specialist skill - maybe a radio man, tank driver or possibly a cook or mechanic.

The technician grades were replaced with "specialist" grades after the war.

ETA: Info here on ranks : http://www.fatherswar.com/8thinfdiv/WW2/ww2_8thgeneralinfo/general%20info/WW2%20Rank.html