P0303 CYLINDER 3 MISFIRE DETECTED

P1870 TRANSMISSION SLIPPING (AS PER CHILTON) /
MFG. CONTROL TRANSMISSION (AS PER OBD)

2002 AVALANCHE Z71

The reason I ran the diagnostics is because my trans. is acting funny.

Going into 3rd was high revs. when it was driveable. (coming home from work) Checked fluid. Needs to be flushed for sure, but was good on the level. Didn't smell burnt.

Got home and parked it. Tried to drive it somewhere to get checked out but it just lost all power (not electrical power, but like it wasn't in gear) and revd high. Let it sit for 5 minutes and tried troubleshooting by putting it in 4wd to see if it may be a differential. Didn't budge.

So, I tried coasting it down to my driveway and eventually got very little power after letting it sit.
After I ran the diagnostics last night I put it in R and it seemed to work fine. I don't want to try driving it due to it's location in case I need a tow.

Tried giving all info here. Let me know if you need more info to help.
Basically, What do i need to check/do? Lost power while driving!!! It was tryin to give me some juice as it went out and was sporadic, giving me a little power here and their.
Thanks in advance.