In superposition theorem any linear bilateral circuit, which contain, G/R of same frequency the current that flows in any branch is the sum of the current's that would result from each G/R Working independently while other G/R. are replaced at that time by their internal resistance if any.
Take one voltage source at a time and replaced the other one with either short or internal resistance.
Determine the particular current by only one source.
The current flowing in R2 due to VS1, (with the help of current division rule)
Take the next source and repeat steps (1) and (2).
The current flowing in R2 due to VS2 (with the help of current division rule)
Find the actual current. Add or subtract the current due to each individual source. If the current are in the same direction add them. If the current are in the opposite direction subtract.
Add the resultant current
Ia = Ia/ + Ia//
Ib = Ib/ + Ib//
Ic = Ic/ + Ic//