Acrobat Javascript - Checkboxes

I'm needing help creating a document-level javascript for checkboxes. Example: if Checkbox 1 or Checkbox 2 are selected, then Checkbox 3 will be visible.

Here is what I have so far:

function MainShop()
{

var c1 = getField("MS 12 CHECK").value;
var c2 = getField("MS 13 CHECK").value;
var c3 = getField("MS CHECK");

if(event.target===c1 || event.target===c2){
c3.display.visible;
c3.display.hidden;
}

}