no 3rd gear???

Ive been driving under regular driving conditions when all of a sudden my car wont shift into 3rd gear anymore. It will shift smoothly into 1st and 2nd and stay in 2nd until i reach about 50mph then go into 4th and its fine. Go back under 50 and it goes back into 2nd. No SES codes come up about the problem. Has anyone had this problem or know why my transmission could be doing this? Im thinking it could be something in the computer maybe? PLease Help!!!

Welcome to AF, post the year and engine please.

its a 1997 regal gs with 116k. I also changed the spark plugs and wires and transmission fluid last summer

The only way to properly diagnose this is have it scanned at a trans shop for solenoid functions, slippage and gear ratio errors.