[DanEC]

OK - if you have the Sunbeam Tiger boot then a couple other areas you might check are to make sure that when you accelerate in 3rd gear, the shifter isn't making contact with the edge of the tunnel cutout as the engine torques over. If you have through-bolted your driver side engine mount to limit torque movement, then this can probably be discounted.

What shifter do you have - a Hurst? The Hurst shifter has stops (bolts front and rear) to limit shifter movement to avoid overstressing the shift forks and internal parts on hard shifts. If you haven't already - it would be a good idea to check the front stop that limits shifter throw on 3rd gear. If it is too tight it might be limiting full 3rd gear engagement.