99 Chevy Cavalier Tranny

I have a 1999 chevy cavalier with 114k its an automatic. When I drive 1st gear goes through just fine 2nd gerar goes through just fine but when I hit the 35-45mph range in 3rd gear it almost feels like its not engaging. The engine will will sound likes it in neutral but will not gain speed sometimes it will engage and sometimes it wont. But if I manualy drop it down into 2nd it will engage just fine. Can someone help me out here? Hoping it aint the transmission.