UPDATE:
After discussions with other collectors, I've determined that, with the exception of the special 20551 set, Gilbert never packaged 3 cars in the same set box. I believe that RoyBoy is correct in stating that the "3rd car" was always packaged individually. The picture that Gandy provided appears to simply be an early 2-car set (notice that the cars include a pair of (247)20 coaches with the (247)30 "add-on" car in the spot where the circle of track should go. A true 3-car set would have had a {247)20 and a (247)40 in the insert cutouts and the (247)30 in its own box.
Evidence seems to show that all the 20550 sets came in the same B2D709 "2-car" box with the (247)30 individually packaged, whether as a "free" add-on in 1959 or as part of a 3-car set in 1960. The Greenberg "Sets" book (p148) implies that the B2D782 box was used for the 3-car sets, but until someone comes up with an example, I'm inclined to disregard this statement.
As always, I welcome any information that proves otherwise. Thanks to those who have provided information.
