2002 chevy trailblazer check engine light went on, took it to autozone they said it was a misfire in cylinder number 3, and that I needed to replace sparkplugs and wires. could this be the problem?"

Your checking for codes & throwing parts at it

Your NOT diagnosing anything like the cylinder
misfires, because you don't have OBD2 System
knowledge or the proper professional tools to work
on it, or Chevy Trailblazer Powertrain experience

Best left to a repair shop

Thats exactly what it sounds like.

Yes, It is possible that the system corrected itself. You may have to wait few days to see if the problem is actually soved.

One thing you didn't say you replaced was the plug wire. That could also be the problem.

The labor sounds like it is in line with the standard labor guide if your Trailblazer is a 4X4. I can only guess how the repair went from oil change to intake valve. Was the "Check Engine" or "Service Engine Soon" Light on when you took it in? Did they tell you that your engine has a misfire that will require the replacement of a valve to fix? If so, this sounds reasonable. Otherwise, I don't know what to tell you without knowing all the details.

Rough idle can be caused by dirt in throttle body or EGR system, but there are many other possibilities. The service engine light indicates the computer has detected a fault. If you can get a free scan at your local parts chain and post the trouble codes to us we can give you more specific repair recommendations.

inspect the sparkplug plug first!! After engine is running for 20 sec (while misfiring), pull your #1 sparkplug and check for fuel. If there is no fuel replace the injector!!If there is fuel, post a comment for more help!!

Could be a bad plug, bad coil pack or bad injector. Try moving coil pack to another cylinder, if not that, move plug.

did you replace only one spark plug? that can cause issues because of differences in resistances between cylinders. also, did the map code stay off after replacing
?

Code P0301
Ignition Coil Pack
Ignition Cable
Spark plug
Clog or dirty fuel injector
Evap canister purge valve
Engine Compression
fuel pressure

you have 3 type of engine for your truck
2.3L 4 cylinder
3.0L 6 Cylinder
4.0L 6 Cylinder

need to know that type Engine

The reason why the service light come on because it is a hard code
not a soft code.

Let me know