Voltage in a Y-Δ transform
There is a quite simple way to solve this (without Y-Δ-transform). Here is an outline:

Recognize that nodes A and B are connected by 3 parallel Thevenin sources (= voltage source with resistor in series).







simulate this circuit &ndash; Schematic created using CircuitLab

Convert each of the Thevenin sources into a Norton source. 







simulate this circuit

Those 3 Norton sources can be combined very easily into one Norton source.







simulate this circuit

If you want, you can transform that combined Norton source back into a Thevenin source.







simulate this circuit

Done.