It's my understanding that the tradidtional holy trinity refers to the prime lens lineup of 24/35 - 85 - 135L. However, when I hear people talk about this trinity, it's almost always in reference to FF cameras.
Perhaps the equivalent on a crop would be 14L/24L, 50L, 85L
All I know is that with my 17-55 IS and my 70-200 2.8 IS (also have a 1.4x T-con), I pretty much am covered for 90% of my shots. I bust out my sigma 30 1.4 every so often when I went less DoF, and my Tokina 11-17 fisheye for cool prespective shots, but I am pretty much all set.
I would love to try out a 1.2 lens though.