So a couple of months ago I bought a '07 Mazda 6 from a private owner and off bat he told me what problems it had but they seemed minor and I didn't consider any of them deal-breakers. One of the problems was that when you shift into 2nd or 3rd it grinds. The seller said the grinding was because it just needed some gear oil.  I drained all the transmission fluid and then filled it back to what the recommended amount was (It called for 3 quarts and it did seem to happen less frequently but I also learned that if I waited for thenm rpms to drop before shifting into 2nd it had no sound, and acted normal.  Well a few days after getting the car I feel asleep and my ex took off in my car for 5 days and when I finally got it back 3rd gear was gone. You can try to shift into 3rd but all does is make a sound like it isn't going into gear. What could be causing this? The transmission seems fine, it is basically just not shifting into 3rd gear. I want to get this fixed but I want to have an idea of what this is gonna run me.