I would join a few mustang forums and read up on them especially mach1registry that will help you. It might just the cable is long and out of adjustment. Are you new to the machs? There is a plastic quadrant already in the car if you didnt know.....if did know I apologize for repeating but reach far up under the dash above the clutch cable and see the plastic adjuster I think they are white. Feel for the tab with your thumb and press hard as heck.....it will move I promise, your thumb will hurt likely but it does move and will feel a significant click.......that will take up slack for you.
From there you can tell right away if helps with that 3rd gear upshift. a nice mgw adjustable shifter will help tons too. Wont know if anyone changed it till taking that cover off if its the stock one angled toward the driver then theres a huge issue....they are garbage!!!!
If that quadrant helps then you know what you need. Now, if you shift hard and let off the clutch fast and it returns with slack it will need to be adjusted again. thats normal get a good one