How to ON/OFF two fans with 3-state rocker switch?
To do what you want, you will need at least one diode in the circuit, between poles I and II, At the expense of having a voltage drop in one of the fans.



When in position I, the diode will not let any current pass trough it, so only Fan I will spin. When the switch is in position II, Fan II will get power, and the diode will let current trough it (with some voltage drop) and fan I will also get power. Because of the voltage drop, Fan I will spin slight slowly than if it were directly connected to the power supply.

For small currents, any ordinary diode will do the job, like 1n4148 or the 1n400X series. Take a look at the datasheets of these models (and the fans, if available) to see the current specs.