I don't have any of my literature out to look up the diode types shown on the schematic provided, but the symbols drawn are for Schotty (hot carrier) diodes, not zener diodes. As such, in the first two circuits, they limit excursions of the signal to around -0.1 or -0.2 volts with no limit in the positive direction. In the second circuit, they clip the signal if it exceeds the window of -0.1/0.2v to +3.4/3.5v. Actual voltage depends upon the current through the diode, and for low currents on a hot carrier diode, the voltage drop will be quite low, around 0.1v
Dean