3 Year Master's Full Time?
Many master's programs do have a term limit associated with them. However, many master's programs may require additional classes, particularly for students coming from other backgrounds. So you should definitely check the specific programs you're interested in enrolling in, to see what is specifically allowed.

But extending the master's or PhD program just to take classes seems like a poor use of resources. One of the goals of advanced education is learning to become an expert in a new subject. After a while, you should be able to get much of the same content that you would from enrolling in a class through self-study.