Moving between universities for PhD studies
It seems that you've got lots of great options available to you - kudos to you!  Given the situation you described, I don't think returning to university A will likely have a detrimental impact on your future.  If your work is good and you get several papers published at competitive journals/conferences/etc., that should matter much more.

The exception here might be if university C is considered far more rigorous than university A.  For example, switching from a state school to MIT or CalTech (though even this isn't a perfect analogy - it's largely field-dependent).  Outside of that though, the university itself will likely matter less than the research you're doing.

And as always - follow the funding :)