96 honda civic dx manual transmission.2nd goes in but pops out and 3rd doesn't go in at all.. when the car is running on idle I can press the cluth in and shift it in 3rd BUT when I let go off the clutch, it grinds and pops it out.. when running I can shift it to 2nd gear but when I let go off the gas pedal suddenly ,it pops it out... what could it be? transmission?

The clutch may be toast.