I got a 2001 Chevy blazer 4wd. When you put the transmission in drive it will shift from 1 to 2 perfect but once it shifts into 3rd it’s just revs as if it was in neutral. I don’t have any check engine codes.

 

So it must not go past 25-40MPH then?

If 3rd gear doesn't catch, sounds like 3rd gear is shot. (Done)

If it's not fluid level (is there enough in there, even overfull, and is it clean or is it black like coffee) or a transmission control (TCC?) then sounds like 3rd is gone..

 

Likely a bad 3/4 clutch pack.