How can i apply extension stay as i want to stay for 3 months? I'm a Singaporean....

Eddie, I am afraid that it isn't easy for you to be a monk in Tibet. According to what I know, Chinese visas are not enough. If you plan to go to Tibet, then you should apply for the Tibet Travel Permit. However, it's almost impossible for you to acquire one unless you join a group tour and ask the agency to help you with the Tibet Travel Permit.