Find current and VAB with Superposition Theorem
Although it can be solved using superposition I think it's much easier to use Thevenin/Norton equivalents:

What you have are 3 Thevenin sources in parallel (i.e. voltage sources that have a resistor connected in series).

A Thevenin source can easily be transformed into an equivalent Norton source (i.e. a current source that has a resistor connected in parallel):
\$R_{Norton}=R_{Thevenin}\$ and \$I_{Norton}=\frac{V_{Thevenin}}{R_{Thevein}}\$ 

Do this for each of the 3 Thevenin sources and you get 3 Norton sources in parallel, i.e. you get 3 current sources and 3 resistors connected in parallel.  

3 current sources in parallel can be replaced by 1 current source whose current value is the sum of the 3 single sources: \$I_{tot}=I_1+I_2+I_3\$

3 resistors in parallel can be replaced by one resistor whose resistance value 
is also easily computed: \$\frac{1}{R_{tot}}=\frac{1}{R_1}+\frac{1}{R_2}+\frac{1}{R_3}\$.