The joke here is that one of 3's readings in Japanese is 'mi', so anywhere you see a 3 you would pronounce it as 'mi'.

Thus KMMM3333MM would probably be pronounced "Kemomimi mi-mi-mi-mi mimi".

A similar bit of goofyness applies to ミ too, which is pronounced "mi" as well. It could stand for the "mi" in Kemomimi-chan's name, or the number 3.

Bonus points are rewarded for the fact that the word for ears in Japanese is "mimi" too.

Mimimimimimimi.

I wish we used this money.