:( I work part time for a telemarketing company from 9am to 2pm, monday through thursday, and now my boss is thinking about adding a second shift from 4pm to 8pm, and a third shift on saturday mornings. Can he make it mandatory for me to work the extra shifts?

Unless you have a bona fide contract that specifically says otherwise, yes. You can be required to work whatever hours the employer wants you to work.

Yes, the employer can add hours. The employer can cut hours, too.

If you cannot work the additional hours, the employer could hire someone to work the additional hours. If that person can also do your hours, that new hire could end up taking your job away from you. Even if you qualified for unemployment (not a guarantee, given your refusal to work the hours needed), that does not come near to being a full paycheck.