If the two plugs you changed were #s 4&5 and they were also changed reciently there might be a problem. Plug #3 may have an internal short to ground taking out the other two plugs. Taking the wires off and reading from the centre terminal to ground should be open. A little more information would help like the plug numbers that were changed before and this time. But if the plugs this time are different than before you should make sure the voltage is going off with car running. Relay points could be welded together is one thought. Anyways more information if you are still having problems. Do not expect a little hairline cracking of the insulators is a problem unless the insulators have become conductive and thats unlikely in my opinion.