is it possible to get the first stay as a 3 month stay on my 12 month multiple entry visa?

You can try to apply for a 12 months multiple entry visa with 90 days stay duration per entry. The stay duration varies from 30 days, 60 days, 90 days. If you get the multiple entry visa, you can stay in China for 30 days, 60 days, 90 days per entry.

Please provide the relationship certificate, the letter from the hospital where your mother in law lives. If you married a Chinese woman, the marriage certificate is very helpful for the application.