3 Phase to Single Phase Transformer
Writing my comment as an Answer (though it is not) as I do not have enough reputation (in electrical forum) to comment.

Last year I tried this by connecting R + Y + (reversed)B phases (R+Y-B) in series on the secondary of 3Ph Transformer. 

As R + Y gives a vector exactly opposite to B, by inverting B we get -B in Phase with R + B. I simulated this at 210V using 3X3Ph Transformers and lamp bank. 

As expected, I got single phase supply (with appropriate scaling) but got disappointed to see Unbalanced Phase currents on primary side. Later rechecking model on paper showed me that my idea was wrong. Primary being 3 Phase it will always load  B Phase (which was reversed) unbalanced. Hence discarded the idea.