The JScript substring() function will split a string of known length. (For example, a Social Security number split into its three-part format.)  The first parameter says where in the string to begin (zero-based).  The second parameter says where to end, going up to (but not including) the end:
 

if (Event.Fields['Employee.EmployeeInsured.SSN'] != null) {

Event.Fields['EmpSS1'] = Event.Fields['Employee.EmployeeInsured.SSN'].substring(0, 3); // begins at the first character (position 0) and goes until the 3rd character (the one in position 2)

Event.Fields['EmpSS2'] = Event.Fields['Employee.EmployeeInsured.SSN'].substring(3, 5); // from position 3 to position 4

Event.Fields['EmpSS3'] = Event.Fields['Employee.EmployeeInsured.SSN'].substring(5, 9); // from position 5 to position 8

}