--I am pretty sure that if you can provide a diagram of the switch operation, and a drawing of how it mounts, that someone will be able to recommend a reasonable solution. Often switches like the one shown are not using all of their contacts, or the contacts are being used in a redundant fashion, and so a simpler switch can, sometimes, be substituted.
First, I want to thank everyone for responding to such an "uninteresting" post.
I've attached a drawing of the electrical circuit and of the mechanical dimensions of the switch.
I don't have the schematic of the device, but from what I understand from the PCB and the panel the three positions are used this way:
Position 1 and 2 select an operating mode (something like auto/manual).
The actuator (I don't have it, was lost when broken) moves only through position 1 and 2 because the front panel's slot limits the travel. You need a screwdriver to operate it.
If you remove the PCB from the panel, you can slide the switch to position 3 (for testing or adjustment, I believe..) So all 3 positions are used.
I'm thinking about using a 2 position switch and hard-solder the pads of position 3 in the "normal" position, but must check carefully the PCB....
Shorting or non-shorting contacts seems not so important, because the device is unpowered when the switch is operated.
Best regards