Is it possible for a sim to have more than one job in the Sims 3?

No it will make you quit your first job even if the second job is a part time one but you could always make money playing the guitar or writing novels in your time off work (writing makes a ton of money with high skill lvls btw, like sometime 15K a week for six weeks if you write a masterpiece)

No, you can still only have one job at a time.

No you cannot. You can however have your teens work part-time.