If I work three jobs is it necessary for my employer to know?

No, you don't need to let your employer know. Many people today are working 2, 3, or even 4 jobs just to get by. How you spend your free time is your business. So you're free to pursue other work outside of what you're doing now.

Now there are exceptions to the rule. For example, if you have a contracted position. What's the problem here? Well many of these contracts have conflict of interest provisions. For example, while you're working on a project for a client, you would be prohibited from working for that client on the side. That's to prevent a conflict of interest and any possible undue influence. But aside from this, you're free to do what you like.