Is ghusl obligatory for a girl who saw a lot of white, viscous discharge three years ago, without being certain if it was semen, and what about her prayers and fasting for that period?

Ghusl is only obligatory upon certainty of the emission of semen. Doubt about what was emitted necessitates choice, therefore, nothing is required of you. We advise you to turn away from whisperings (waswas) as indulging in them leads to great evil.