"T3" is MOSTLY a flange/physical size description. Wheel sizes would be a better indicator of how they would work.
Going rear mount would be a good fit for the smaller exhaust size that a T3 has.... But I think you are asking a lot here from the turbos. "no lag" is not going to happen with rear mounts.
Back pressure is not 100% from exhaust wheel/housing sizing. If the compressor wheel is to small, the turbine wheel spins faster... To the point that the turbine wheel inlet pressure is rising due to centri-force.
I have personally experienced going with a larger compressor- lowering back pressure dramatically. This is why you see T3/T4 hybrids.