I have a three year visa for mainland china and i am from Nepal. what should i do to go Macau???

Dear, yes, you need to apply for another entry permit to Macau.
First, prepare the relevant required documents: the filled-out application form, your passport and copy of its data page, proof of your financial support, transportation arrangements and accommodation reservation, proof of travel purpose, as well as your residence permit in mainland PRC.
Next, submit your application to the Office of Macau SAR in Beijing and wait for the result.