On your input side you only connect your 3 phase feeds. You do not connect any sort of neutral to the transformer primary. If your transformer has common terminal names, then you connect nothing to 'X0'.
For 'corner grounding', you use all three phases. But one of those phases is connected to ground, becoming the 'grounded phase'. You don't cap it off, you use it.
Consider an ordinary residential 120/240V single phase service. You have _3_ wires that come from the transformer, and you supply your loads with various combinations of those three wires. One of those wires is the neutral, and it is _intentionally_ connected to ground, and is called the 'grounded conductor' or 'neutral'. If you want to supply a 120V load, you run one 'hot' and one 'neutral' to the load. The physics of the situation would actually allow _any_ _one_ of the transformer terminals to be grounded...but it would be quite confusing if you had 120V from 'hot A' to 'ground' and 240V from 'hot B' to ground
For corner grounding, we embrace the confusion. You have three transformer terminals: H1, H2, H3. To supply a three phase load you need to have a current carrying path (wires) from H1, H2, H3 to the load. When you 'corner ground', you take _one_ and only one of those terminals and also connect it to ground. Say you connect H2 to ground. The voltage from H1 to H2 is still 480V, the voltage from H2 to H3 is still 480V, and the voltage from H3 to H1 is still 480V. But now the voltage from H2 to ground is _0_ V. The three phase load still needs the connection to H2, and that wire is still a current carrying conductor...but it is simply a _grounded_ current carrying conductor, and it is _not_ a neutral.
-Jon