Summing Integrator cut off
I would like to know how to calculate cut off frequency of this
summing integrator circuit.

Integrators ideally don't have a cut-off frequency: -

Picture from this answer.

Then how do I calculate unity-gain frequency this circuit?

For each input, calculate \$\tau\$ where \$\tau=C\cdot R\$ then, take the reciprocal. That answer will be in radians per second so, to get it in hertz, divide by \$2\pi\$. With different input resistor values there are different unity-gain frequencies for each input.

Picture from this slide-player