a) being EU you do not need to apply for a permit for them. All that's needed is for them to have an employment contract.
b) however 3 hours a week will certainly not be enough for them to get a permit. Minimum would be 50% of the working week - 7 hours a day iirc so 17.5 hours a week. Unless you're paying exceptional wages for those 3 hours of course.
Check with your migration office on the minimum hours/salary they would accept. And yes the employee can seek other work providing you're happy for them to do so. But they won't get a permit with just an employment contract from you - the hours aren't enough I'm afraid.
If they're willing to risk it they can move here and ask for a job seeker's permit for the initial 3 months. Depending on how quickly you need them that would give them some time to line up other work on top of yours to make up the necessary hours/salary the migration office will expect. However, their chances will depend on what skills/experience they have to offer, whether they speak a Swiss language, etc. They may find it easy to get additional work or they may not. They and you need to think carefully about this before they make the move.
Have a look at these websites which will give you a bit more info on permits, etc. www.ch.ch https://www.bfm.admin.ch/bfm/en/home/themen/arbeit.html