When I make triple, I stick with just pils and sugar, a la Westmalle. After all they invented the style.
For me, mixing base malts just makes a modified base malt. Not all that out of the ordinary but if I want to clone Westmalle I’d probably stick with Pilsner/Pale Ale. My inspiration is always Westmalle Tripel but I also add some CaraHell. For Singles and Tripels I don’t use Vienna in the blend.
For Dubbel and Dark Strong I blend in Vienna for my custom base malt and use a few select specialty malts: CaraBelge, CaraMunich II, CaraBohemian and Special B. Not all in the same recipe obviously.
I really love using 3787. It’s probably my favorite yeast.
EDIT: With all that said, I don’t go nuts with any of these four beers (All I brew anymore is Single, Dubbel, Tripel, and Dark Strong) and pay extra attention to fermentation. I really just use 3787, 1214, and 1762 based on what I want, although I have used 3787 exclusively for all of them for a while.
I limit lighter beers to Pils/Pale Ale/CaraHell, with Singles getting light spices (Coriander and Sweet Orange) and Tripels getting sugars.
For Dubbel and Dark Strong Pils/Vienna as my base, sugars in both, with Dubbel getting a mix of 2 lighter Cara malts, and Dark Strong getting Special B. I don’t use anymore than 2 specialties and only in Dubbel.