Let me see if I'm understanding your issue correctly. You say the engine will not spin over 3500 in 3rd gear? This occurs even if you have the accelerator floored? Once you have the chance check to see if this is also happening in "Normal", "Sport", "Track", and "Wet/Snow" modes. Also try to manually shift it with the paddles and see if that forces it to shift from 3rd or 4th. There may be an issue with your TCM (Transmission Control Module).
2016 Ecoboost Mustang
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2005 Mustang V6 (Sold)
2003 Terminator (Sold)
2002 Mustang GT (Sold)