As mentioned by brokenhead, one hindrance to the use of triples seems to be some enthusiasts have encountered clearance problems with the rear carb.
We have done a number of triples for both Chevrolet and other USA sixes, always with good results. However, I don't remember any of these going into trucks.
As to linkage: Jim, you mentioned that you worked on a number of imports with multiple carbs. How many Jaguars or Healy 3000's with triples have you worked on that used progressive linkage?
Jon.