3 band audio equalizer circuit transfer function
This cannot be reduced to inverting amp with gain -(Zf)/(Zs). The RC circuit has three ports. But virtual ground simplifies the case a little. One of the ports is grounded. You should calculate with complex variable phasors which output voltage pulls the inverting input of the opamp to GND. The result will be an equation between Uout and Uin.
If you write the usual node voltage equation (=KCL) for the node where R1, R2, C1 and C2 meet you have the essential law for the circuit. You must eliminate the voltage V2 of that node. It can be done by setting the current through Rf same as through C1. Then you can replace V2 with -Uout/(sC1Rf) and have only Uin and Uout in the equation.