1997 Chevy Tahoe P0303 code - Need Help!

Hello! I am new to this forum and appreciate all of the help I can get. I am trying to get my wife's 97 Tahoe smogged. The Tahoe has 150,000 miles on it with a 5.7 Vortec V8. It actually passes the tailpipe sniffer, but the Service Engine Soon light keeps turning on and won't pass because of it.
Before the Tahoe was taken to the smog shop, I replaced plugs, plug wires, distributor cap and the rotor. I disconnected the battery cable to reset the computer. It looked like everything was OK until my wife drove it about 15 miles, and the service engine light came back on. So she took it to the smog shop to have it checked out. He ran it on the dyno and tested the emissions. They were on the higher side, but would pass. He then pulled the codes. It had a code indicating that the second O2 sensor on bank 2 was bad. It also had a P0303 code indicating a cylinder 3 misfire. I changed out the O2 sensor replaced the #3 plug wire again. Disconnected the battery to reset the computer, and again the service light was off. She took it back to the shop where the light came back on. The O2 sensor was clear, but the P0303 code was back on. He also did a compression check and found that #3 cylinder had 125 PSI. I believe that is enought, maybe not great, but should be enough. I replaced the #3 sparkplug tonight and drove about 15 miles before the service engine light came back on. Any ideas?
Thanks,
Ron

I would say if the plug and wire is new, then you could possibly have an injector not firing properly on cylinder #3..I would have that injector checked

I replaced injector #3 with a new injector - no change.

well, you might start looking at the map sensor, Take out the EGR valve and apply human pressure, see if it leaks in closed position, (wrap your lips around engine side of valve and blow) any slight leak here or sticking is bad, these are just backyard mechanic tricks, mabey replace all the o2 sensers, man I feel for ya, I kinda having the same problem with my baby 03 hemi po132 truck, I used to live in a smog dmv area and I hated the issues of all that crap, good luck, maby some slick dealer tech will come on here soon and have a fix for both of us

for the dodge check your dealer for tsbs, for the tahoe check the forums for gms