Some of the household Royals did use a 3-wire cord but very few of them (less than one percent) would have been specified with such (3-wire cords are used mainly on commercial-duty models). A household-grade Royal with a 3-wire cord would have a "T" appended to its model # (e.g. 882T, 8801AET, 89130T, 89135T, 4000T) or, in the early days, a "3-" prefix (e.g. 3-153, 3-501, 3-602, 3-604, 3-801).
~Ben