Assign a Value when two checkboxes are checked

I am trying to come up with a JS for a memory test. In a series of 6 tries, Once the tester selects two consecutive checkboxes, I want to assign a Value of "1" to a specific field.

To simplify: if checkbox1 && checkbox2 are checked, then assign value of 1 if checkbox2 && checkbox3 are checked, then assign value of 1 just to checkbox2 and checkbox3

Any help will be greatly appreciated.