Checkbox controls in an ascx file

I'm trying to use 3 checkboxes in an ascx file, that is linked to an aspx web form. I want to test for when all 3 are checked. I'm using the checkbox_checkchanged event to toggle an int variable that keeps track of whether a checkbox is checked or not. I've found that even though all three boxes appear to be checked, there is never more than one that actually has its Checked value set to true. What can I do to overcome this?  Thanks.