It hard to see, but the terminals on each end of the 3 are actually slightly curved to produce Mickey's head.
I took a walking backstage tour of Spaceship Earth, and this was pointed out by our guide. The artist who created the entire mural placed it in himself, and manipulated the 3 to make it work. I know it may look weak, especially from a distance, but I guarantee you it's there.
Ah, so that's where it is! I stared and stared as long as I could (the line for SSE went pretty fast the last time I went to Epcot), and did not see that!
I agree it's not as clear an HM as it could be -- the artist could have curved up the ends of the 3 a bit more -- but I don't doubt it was intentional.