1993 Chevy Cavalier Transmission won't engage 3rd gear in 3

At first when the problem occured, it would not engage 1st gear, unless I put it there myself so it would just stay in 2nd the whole time. I'm not really sure when this all occured exactly, there were no warnings I didn't feel anything strange. Me and my dad/mechanic don't think there's really anything mechanically wrong with the transmission, and might be a pretty easy fix. The transmission is a rebuild, I learned through some research on the net, but it has only been at most 35-40k since the rebuild. There's also no slipping. It's better now, it will engage 1st and shift farely well into second, but third gear still isn't there. Also, when I mash on the accelerator in second going say 15-25 it will not shift down into 1st. There was also some gas pedal spongyness/resistence about half way through acceleration, but that was fixed when my dad had been tugging on some cables(pedal, transmission cable) whatever cable had it clicked twice and I went and drove it and the pedal feels fine now, but still no shifting. Any help guys?