If your Mustang has a standard shift, It's probably the throwout bearing or the synchronizer.
Why Does My Car Jump Out Of 3rd Gear?
It could be many things,loose or damaged shift linkage or build up of dirt in between the transmission housing or it's loose or out of alignment. Have an appraisal done by a mechanic and get it in writing before having any work done.hope this helps.
The tranny is going bad