Gary,
The NRECA Simplified Staking Manual for Overhead Distribution Lines has two tables on pages 75 and 76 with recomended minimum pole sizes for transformers installed on a single pole. For example, they recommend a minimum of class 3 for three 50 kVA transformers. I don't recall RUS bulletin 160-2 having a table, it just presents the methods to calculate pole size. In short, the pole size is a function of transformer weight and other stuff attached to the pole, and wind loading on the pole, transformer, wires and other stuff attached to the pole. Please let me know if you need more info.