Manual Transmission - Synchro issue?

What does it mean when I can turn the crank almost freely with a breaker bar and socket while the trans is in gear? It only happens in 3rd gear though.

Sounds like broken teeth. How does it drive in 3rd?

Drove it today and if I hold it in third it's fine but as soon as I let got it pops out.

3rd gear synchro keys or fork pads or both are shot. Time to rebuild assuming you dont have an errant shifter stop...