You could jack it up with the wheels off the ground and disconnect the 2-3 shift rod. Put the lever in 3rd gear, start it up and see if it will stay in gear.
If it still pops out, you could have a worn syncro assy, shift fork, or both.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine