Input Output Relationship of Inverting amplifier
The voltage at the i/p of the op-amp is not \$V_{in}\$ but the voltage across \$R_2\$. By considering virtual ground, \$R_2\$ and \$R_3\$ are in parallel and total current supplied by the source can be given as \$\frac{V_{in}}{R_1+R_2||R_3}\$. Use current division and find the current through \$R_2\$. Once you find the voltage across \$R_2\$, find the o/p by using the gain for an inverting configuration.