I have a three-line set-theoretical definition of modus tollens that I want centered on the line, each aligned below the one above. The AMS-multiline produced the first line left-justified, the second line centered and the third line right-justified.
Here's what I'm entering:
A \subseteq B x \notin B \therefore x \notin A
If I have correctly understood what you are wanting, I think that you can use the gather environment to obtain it.
I hope it helps you.
Paul
