2002 Avalanche Z71 - P0303 and P1870: Transmission Slipping & Cylinder 3 Misfire Diagnosis

My 2002 Chevrolet Avalanche Z71 is showing two diagnostic trouble codes: P0303 (Cylinder 3 misfire detected) and P187-70 (Transmission slipping, as per Chilton's manual and OBD-II manufacturer control). I ran diagnostics because the transmission has been acting erratically. When shifting into third gear while driving home from work, it was running at high RPMs even though the vehicle was drivable. The transmission fluid level is okay and doesn't smell burnt — however, I believe a full flush is necessary. After arriving home, I attempted to drive again but experienced sudden loss of power (not electrical, but mechanical — as if the vehicle wasn’t in gear). The engine revved high without any forward movement. I let it sit for five minutes and tried shifting into 4WD to check if the issue was related to the differential. No response. After coasting down to my driveway, power became very inconsistent and minimal after sitting idle. Last night, after running diagnostics, I shifted into reverse (R), and the vehicle responded normally. Due to its location, I’ve decided not to attempt driving it further without professional assistance. I’m seeking advice on what to check or do next — especially regarding the sudden loss of power while driving. The issue was sporadic, with intermittent power delivery at times. Please let me know if additional details are needed for a more accurate diagnosis.