Hi
I have no clue how to serial connect those three sensors to one PLC. It would be much easier if i had 3 NPN or 3 PNP, but it's a theoretical question for my studies. I'd appreciate it if someone explained it to me and check if my thoughts have any sense.
I doubt that somebody really does it this way.
Connect all "+" to plus supply
Connect all "-" to minus supply.
Connect each sensor output to an independent PLC input.
Depending on output signal polarity and expected function you may use "wired AND" for all NPN outputs.
Or "wired OR" for all PNP outputs.
But it is impossible to give a general answer without knowing function and polarity.