My husband is doing P.h.d IN Beijing. Which kind of visa is best for me to stay for 3 years.

You can try to apply for a S1 to stay with your husband. After reaching China, you need to change your S1 to a resident permit, so that you can stay in China for at least 180 days. You only need to renew your resident permit to live in China legally.

And can you please guide me that can i do job in Beijing while living there?
My qualification is bachelor in chemical engineering. And in Pakistan I'm currently teaching mathematics to grade 10 in well reputed school.
What should i do in beijing as job? Will i be able to get?

If you hold a S1, you are not able to work in Beijing, because it is illegal.

You can try to look for a job in Beijing. After you get the job, you can change your S1 to a Work Z.

For what to do in Beijing, it is unclear about the job information for me. You are suggested to enter Beijing first and search online. Or you can ask your husband or your husband's school to help you to find one.

Thank you so much :-)