They are mostly the same but there are a few differences. A job is what you do day-to-day. For example, your job may be as an office worker, a grocery clerk, or a doctor. This jobs can be unrelated and you can switch fields, for example from office work to construction. A career has a longer-term sense of what you do. For example, you can make a career in law working at different companies. Even though your job changed (different title, different responsibilities, different locations), your career path (law) stayed the same. An occupation is used much less often. I'd say it is closer to career in that it refers to the career you work in. For example, if someone asks your occupation you could say "I'm a plumber" without having to mention your exact company/title.

As Andrew said above. "Occupation" is used more frequently on a form to fill out than in speaking, and means "job", whatever work you are doing now that pays, or it can mean "career" in that you might still put "computer programmer" even if you are out of work at the time.

A job can be either a piece of work, or an activity for which you are paid. A career is a lifetime progression of paid occupation. A profession is a career with high status and one requiring training, often at degree level. Occupation is usually a paid job, whether professional or not.