Assume by/by way of /toward contradiction

Mathematician here. The 2nd item in your list of suggestions is by far the best. The other two sound a little strange to me, and may very well be grammatically incorrect.

I will suggest a similar phrase: "Assume, for sake of contradiction, that...." I use this in the courses I teach, and my students and I end up using the acronym AFSOC as a shorthand.

I like this phrase because it accurately conveys what is going on in a proof by contradiction. That is, it really says, "I'm going to assume this is true right now, not because I really believe it, but because I will use it to find a contradiction."