So these are the signup options correct?
1. General List signup. no Events or Volunteer needed.
2. If Events is selected, no General list signup but Volunteer can be selected, too.
3. If Volunteer is selected, no General list signup but Events can be selected, too.
This should be possible with some JavaScript, however, it might be tricky considering all of the checkboxes have the same name (the HTML lists[] array). This is the only reason I didn't provide an example just now..
You could also use radio buttons so that only one can be selected at a time. This might prevent confusion.