The illustration is showing three single phase transformers connected in wye or delta, not a 3 phase transformer. They connect them in school that way so the students actually make the wye and delta connections and they can see how the voltages add and interact.So all the high-side phases are all H1, the neutral is H2, and all the low-side terminals are all dual labeled X1/X2.
Can't wait to see the guys apply their book-learning on the next transformer they get to wire. Apparently apprenticeship courses still leave a lot to be desired.
To the OP, the open corner test shows that there is no potential between the last two points before you close the delta. If it's good, you see zero volts. If you read double phase voltage you've got one winding that is connected backwards.