Mike, I don't think the OP is planning to multiplex the display. I think the design is to use 21 x 4017 (in 7 groups of 3 cascaded 4017), but so far the prototype only has 3 x 4017.
21 chips is certainly not the way I would have designed it either. I might have used multiplexing, or rather used chips to do the multiplexing like 4 x max7219 or 2 x ht16k33. Or maybe I would just have used ws2812 led strips.