Instead of attacking normally, he inflicts D3 hits on his own unit when it is his turn to attack
I take 5 MSS tests, fail 3 and then when it is my time to attack I do D3 hits on my unit.
I've followed every rule word for word and not done multiple sets of D3 hits on my unit. You can fail many tests and the result is that "he inflicts D3 hits on his own unit when it is his turn to attack". Nothing says that multiple fails cause you to do multiple sets of hits, so fulfilling the requesits for 1 fail will fulfill them all.