Yes P0303 = #3 cylinder running lean but first here are the basic conditions:
This is a Surefire reman engine with new secondary ignition. The problem started with the installation of the motor. The mechanica called and explained the injectors needed replacing.
I took it home and replaced the old poppet valves with a set of the new Delphi injectors. Same problem P0303.
I thought I had screwed up the installation and got another set of injectors. Same result P0303.
I don't know GM products so I am lost as to where the problem could be, a broken wire between the computer and injector plug?
A bad computer? Are there any other components likely to cause this condition?
Thanks for reading this.
Ubbm.

Did you check compression on cylinders 1,3, and 5, and compare? Did you check spark on that cylinder? Did you try swapping a spark plug with that cylinder and see if the problem moved?

Cylinder running lean?

P0303 is a misfire on cylinder 3.

Switch the spark plug with #5. If the misfire doesn't move, switch plug wires around. Years ago, I did a tuneup on a 4.3 and immediately had a misfire on #1. Come to find out, brand new spark plug had a crack in it.

Thanks for suggestions.
I replaced #3 spark plug with a new one and the problem remains.
I have replaced the injectors which is what the motor installer claimed was the problem. I think at this moment I should return the Blazer to the repair shop and have them diagnose the problem. I have paid them $1200.00 labor for the installation of the reman engine. I have $4500.00 in this replacement motor so far.
I am getting too old for this "stuff'!
I'll report what they find.
Thanks,

Did you switch sparkplug wires yet?

#3 injector wire to pcm could be broken. would have to be open

Sounds like a compression test needs to be done.

Here is the answer: Apologies for such a delay, life gets busy. The repair shop replaced the #3 injector and all is good!
Whether the previous injectors were bad or I screwed something up, we will never know.
Thanks for your help.