I just did a little research into LLL.io and it might sell out like the other 3L TLD and some gTLD's, regardless of the high reg fee. Out of the 17,500 possible combinations, I looked at the first 5000 and I would say that between 10-15% are left for registration. Then I just looked at actual 3 letter words (less than 700) these are all gone along with most combination patterns. Ontop of that, I also checked out 3N.io and there are less than 500 out of the 1000 left to hand reg. So the potential is there for a sell out, but it's going much slower because the founders of .io are charging way too much. I think a Reg fee shouldn't be more than $10.
Disclosure: I do not own any 3L.io and only 1 3N.io (However I am contemplating buying some 3L and more 3N)