Perhaps I'm confused (happens a fair bit to be honest) but am I the only one thinking that one number and two letters will give you even less callsigns than three letters, there being 26 letters in the alphabet and only 8 available digits (not counting 1 or 0 as mentioned above)?
Ahhh... disregard, I see now that they can come in any order, so there's heaps more permutations - or is that combinations?