i was researching this a while back and what I got is that 3 valles is not great for kite-surfing. There are few spots though (though these are not famous, nor great). Seems you're better off just snowboarding/skiing.
http://forum.kitecrowd.com/snowkiting/v ... 90006.html
i LOVE snowkiting but I wont even bother taking my gear there.
when are you there? Im there jan10-16th and i'll be staying in Val Thorens.
IF, however, you are driving there / will have a car there is a GREAT place! Its in resort called LA ROSIERE and its some kind of a valley where its always windy.
i was there 2yrs ago and it was GREAT! Google la rosiere snowkite. The place is called something like PETIT SAN BERNARD or what. You can google the spot very easily (how to get there once your in La Rosiere).
La Rosiere is 87km from Tres Vales.
I was staying in TIGNES at the time. Public transport + taxis was WAY too expensive, but i still did it. that day cost me eur200 (missed my bus, had to take taxi home)
)) lol. Was worth it though...