As of 2016-03-18 all 3-digit .io domains have been registered. Huge numbers were registered in December and January, and then the last remaining 7 were taken on Friday.
According to my records the oldest NNN.io domain is
520.io, which was registered on 2006-09-12. It is actually the oldest .io domain containing only digits.
88.io was registered a couple months later on 2006-11-24, and
100.io was registered next on 2009-03-23. None of these load actual websites - they either do not resolve or show server error pages.
The last ones picked up on Friday were: