I've always heard photographers that praise a photographer for collecting either a 35/50/85 or 24/50/135 lens collection.Point of order - the phrase 'holy trinity of primes' usually refers to the 35-85-135 combo.
I always thought it was 24-50-85 for the Holy Trinity. In reality, any three of 24, 35, 50, 85, 135 could be considered, IMO.
I like the idea of 28 f/1.8, 50 f/1.4, and 100 f/2. I like my 35 f/2, but IQ is not nearly as good as the 50 I, or 85. Additionally, focusing is a joke compared to the 85 (similar speed to 50 I). I had the 28 2.8 for a while, and it's focusing speed was surprizingly fast for non-USM (likely due to the really short travel of MFD->inf) and noticably quicker than the 35 2.
I think 20 f/2.8, 35 f/2 (EDIT: or 40mm f/2.8 STM) and 85 f/1.8 could be a good set as well. I would not count the 24 and 28 IS lenses in with the unholy trinity, due to their near-L cost (even if their IQ is up there with current L lenses).