That is what I thought at first, but there is no other letters or numbers that could be represented by the ice anywhere in the image. I think it was intentionally put there.I dunno, I really think that it has nothing to do with a 3 there. Its just that these icy paterns have way too many shapes if we look too deep on all the possibilities that the cracks and forms can show we will end up refering to way too many things.
Plus if there was a 3 there, the way the font is "drawn" is just out of match with any kind of font that Blizzard has used on their games' title. And Certainly Diablo 3 would come with ronam numerals.
I just think its just one of the many shapes that the icy patern end up creating. The only real plausible clues we have are the runes and the glowing crack in the enter.