You can apply for China tourist visa in Thailand in March/April, which allows you to stay in China in May/June. It takes 4 working days for processing, from the date of issue, a single-entry tourist visa shall be valid for 3 months and permits 30 days stay duration in China, I guess that it is hard to get a 90 days stay duration, but you have a try, write 90 days on the "Your longest intended stay among all entries of your inteded visits in China" on the application form. The final decision is up to the visa officer.