Phil,
First thing I would check is the linkage from the gear slector to the selector lever on the side of the box,. As 3rd gear position on the quadrant is at the end of the gate, it may be there is a bit of wear or slack and the lever on the box is not quite in the correct position. Also you say that when 3rd does engage it comes up with a bang, is it then hard to push the pedal down again? Just trying to ascertain the problem.
regards, Richard