Is ghusl obligatory after masturbation that did not result in the emission of semen, and is ghusl for menstruation sufficient in its place, and what should be done regarding the prayers and fasts missed over three years if ghusl is obligatory?

As long as you doubt the emission of semen, no ghusl is required of you. Ghusl is only obligatory when there is certainty about the emission of what necessitates it. You are not required to repeat your prayers. And, assuming ghusl were necessary due to semen emission, your ghusl from menstruation would suffice for both menstruation and janabah (ritual impurity).