Finding Thevenin Equivalent voltage and resistance
You are over complicating a simple problem. The Thevenin equivalent voltage is calculated with A and B open, so there is no current over R3. The Thevenin equivalent resistance is done making all the voltages passive.

So:

Vth = Vin x R2 / (R1 + R2)

Rth = R1 // R2 + R3   (For Rth, R1 and R2 are in parallel)