Hi,
I don't think the developer forum is the right place for this, as you don't need to do any development to do what you're asking. That said, it depends on which version of Moodle you're using as to how you can achieve this.
For Moodle 1.9 (and maybe earlier versions - I'm a relative newcomer to Moodle, so I'm not sure), one option is to use meta-courses - set up a standard course for each of the groups and assign students to them, then set up your other courses as meta-courses, specifying whichever of the main 3 courses you'd like to enrol as child courses. You can hide the child courses if you don't want the students to see them.
If you're using Moodle 2.0, you can still use meta-courses, but you also have the option of using Cohorts, which are probably a better option in your case (unless you want an individual course for each of the three groups as well, in which case you might want to use meta-courses) - assign the students to three Cohorts, and then use the Cohort Sync enrolment plugin to enrol those Cohorts into your courses.