The way I understand it, it's a printing mistake.
Way back when they did the designs by hand with ink and paper the graphics department would submit multiple samples to the advertising department to choose what to use in the catalog. Usually there were between 3-7 options. These were handed to the graphics design assistant, who would stamp them (you know with one of those big stamps that goes ka-chunk) with a different number at the bottom of the page. These were then passed along to the advertising assistant who would put them up on the light board for perusal by the senior advertising staff. They would mull over them for several days and finally one was chosen by number.
The chosen drawing was passed along to the printing company to include with the catalog and subsequently to create the decals. As fate would have it, a new advertising assistant had just started on the day the choosing was completed. The number 3 (which happened to have been stamped in exactly the center of the fin) wasn't removed from the artwork that was passed to the advertising company. Once the number 3 made it into the catalog, Estes was obliged to create the decals to match (we know how delicate the sensibilities can be of hobby rocket modelers when they can't match the decals to the artwork).
I can't verify the veracity of this explanation as it was passed along to me by the grandson of the best friend of the nephew of the mailman who's route stopped two doors down from the aforementioned advertising assistant. The assistant would regale the mailman with tales of his early days in the model rocket business.
