Can I apply for postal service if I major in computer science with three majors?

Yes.

The postal service usually recruits full-time undergraduates or above. Some provinces (such as Guizhou) will recruit full-time candidates with three or above. For rural outlets below the county level, applicants with a college degree can be relaxed. The recruitment scope of postal majors is relatively wide. Basically, most majors can apply, mainly focusing on: computer, transportation, mechanical and electronics, business administration, e-commerce, financial economics, etc.

China Post Group Corporation is a large-scale state-owned enterprise established in accordance with the "Law of the People's Republic of China on Industrial Enterprises Owned by the Whole People" based on the operating assets and some enterprises and institutions owned by the former State Post Bureau.