with the rockers still on, it would be normal to show 100% leakage on #3 (or any of em) if you were not on TDC #3 compression (it'd leak past the partly opened valves) & on the other hand if the rockers were off, you could still get a good test no matter where the piston is at even with the air haveing pushed the piston down when you apply the air which as you know it would do if you were not exactly at TDC. As said post the outcome. EDIT a cyl not firing would not cause it to read zero & I'm assuming the rockers are still off so you might pipe in 100 psi & see what % you get on em or if the rockers are now on get each one to TDC compression & repeat the test.