For your consideration, Austin Homebrew has a clone kit for this beer where they list Wyeast 3522 and WLP570 as yeast strains. http://www.austinhomebrew.com/Beer/T-V/Tripel-Karmeliet-18C.html#.Vi0FsCuGNNg Cheers!
I've used both 3522 and 3787 for my Karmy Klones . . . in blind tastings, the Ardennes strain wins by a (nose) hair.
What about Wyeast 3739-PC Flanders Golden Ale? There's some question as to its origin - most speculate it's from Brouwerij Van Steenberge (piraat) but neither the brewery nor Wyeast have ever confirmed this to my knowledge. Wyeast simply said they can't divulge the source. It certainly *could* have originated in another brewery in Flanders, like Bosteels... Who knows. In any case, it's well suited for brewing a tripel and - if nothing else? At least it's from the same region, geographically (unlike the Ardennes strain - which comes from the opposite side of Belgium). Food for thought...
