In English, the test of the if is broken into two parts: if (A OR B) where A is "if no boxes are checked at all" and B is "if more than 1 box is checked". If either of these cases is true, then it is not the case that one and only one box was checked, so we issue and alert and return false. Otherwise (else), we thank the user and return true.

if ((!document.easyform.ghost[0].checked 
     && !document.easyform.ghost[1].checked 
     && !document.easyform.ghost[2].checked) 
    || ((document.easyform.ghost[0].checked 
         + document.easyform.ghost[1].checked 
         + document.easyform.ghost[2].checked) > 1)) {
     alert("Hey, please pick one and only one ghost answer.");
     return false;
} else {
     alert("Thanks for answering the ghost question.");
     return true;
}