You could use a Zener diode of, for instance 3V plugged to a negative voltage rail of -2.7V.

You could increase the gain of your summing amplifier then use a diode, then place a voltage divider.

You could also use a mosfet with a feedback from the output voltage that drives or not the fet.

You could use a transistor as an amplifier clipped.

I can think of many circuits that could do that but without knowing exactly your issue, my only recommendation is don't work with such small signals! All the components will bring you significant voltage tolerance. I would definitely try to amplify it before conditioning it.

You could use a Zener diode of, for instance 3V plugged to a negative voltage rail of -2.7V.

You could increase the gain of your summing amplifier then use a diode, then place a voltage divider.

You could also use a mosfet with a feedback from the output voltage that drives or not the fet.

You could use a transistor as an amplifier clipped.

I can think of many circuits that could do that but without knowing exactly your issue, my only recommendation is don't work with such small signals! All the components will bring you significant voltage tolerance. I would definitely try to amplify it before conditioning it.

Edit: Not all those solution will work well in any circuit. My point is without knowing more about the circuit, there are many ways one can accomplish OP's goal.

You could use a Zener diode of, for instance 3V plugged to a negative voltage rail of -2.7V.

You could increase the gain of your summing amplifier then use a diode, then place a voltage divider.

You could also use a mosfet with a feedback from the output voltage that drives or not the fet.

You could use a transistor as an amplifier clipped.

Working with signals in that order is tricky. All the components will bring you significant voltage tolerance. I would definitely try to amplify it before conditioning it.

You could use a Zener diode of, for instance 3V plugged to a negative voltage rail of -2.7V.

You could increase the gain of your summing amplifier then use a diode, then place a voltage divider.

You could also use a mosfet with a feedback from the output voltage that drives or not the fet.

You could use a transistor as an amplifier clipped.

I can think of many circuits that could do that but without knowing exactly your issue, my only recommendation is don't work with such small signals! All the components will bring you significant voltage tolerance. I would definitely try to amplify it before conditioning it.