Verification of Superposition Theorem 
Theory
If a number of voltage or current source are acting simultaneously in a linear network, the resultant current in any branch is the algebraic sum of the currents that would be produced in it, when each source acts alone replacing all other independent sources by their internal resistances.
Circuit Diagram:
Fig.1 : Circuit for analysis of Superposition theorem
Fig.2 : Circuit with only V2 short circuited
In given figure 1 apply superposition theorem , let us first take the sources V1 alone at first replacing V2 by short circuit as shown in figure 2.Here,
$$ I_{1'} = \frac{V_1}{\frac{R_2*R_3}{R_2+R_3}+R_1} $$
$$ I_{2'} = I_{1'}* \frac{R_3}{R_2+R_3} $$
$$ I_{3'} = I_{1'} - I_{2'} $$
Fig.3 : Circuit with only V1 short circuited
$$ I_{2''} = \frac{V_2}{\frac{R_2*R_3}{R_2+R_3}+R_2} $$
$$ I_{1''} = I_{2''}* \frac{R_3}{R_1+R_3} $$
$$ I_{3''} = I_{2''} - I_{1''} $$
As per superposition theorem,
$$ I_{3} = I_{3'} + I_{3''} $$
$$ I_{2} = I_{2'} - I_{2''} $$
$$ I_{1} = I_{1'} - I_{1''} $$