It's a T-5, 3rd gear is notoriously bad in those transmissions. If your clutch is not engaging fully or your fluid is old it could be as simple as that though.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.