At best, you'll wind up stressed and frustrated. At worst, you'll lose your confidence and drop out. Even if you're not working, or working only part-time, you're better off starting with only one, or at most, two courses. It's worth the extra time upfront to consult some of the books available to guide you through preparing for exams, writing term papers, conducting research and time management. And you shouldn't minimize the value of student orientations.
Planning is key.
You will be most successful if you take the time to do proper planning at the beginning. Here are a few of my guidelines. Classroom time is just a small chunk of the total time you'll need to set aside. Expect to spend at least three times more for study, research, homework preparation. Personal rhythms are important.