My firebird keeps reading po303 and jumps from po302 and po301

Its a 3.8 v6.spark is ok and no leaks we have found yet..and compression is ok it was throwing this coode before the head gasket change so the problem was there before we even touched it.

When it started is irrelevant, is it missing on those cylinders? Running rough, idling rough? Forget about codes for now! Get to them later.

Very slight rough idle..we took the num3 wire off to see if it would be worse of a idle and the car started and then just died..is there any way ro test the ecm thats under the coil packs.