Popping out of Gear

[GRN3500GT]

Ive searched and found a few things on it but nothing that really answers my question. Here is what is happenening. My 5 speed goes through all the gears fine except for 3rd. When I go to 3rd from 2nd the only way it goes in with no problem is by pressing the clutch all the way down putting it in third and slowly releasing the clutch then after 1 second or so I can hit the gas. If I shift normally it pops out. the only way to shift normally is to put it in 3rd and holding it until fully in gear for about 3 seconds and sometimes it takes it and sometimes itll grind as soon as I hit the gas and then spit it back at me. What do you guys think it is?