How to detect 3 way slide switch positions?
I think you're misunderstanding how the switch operates.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Each pole of the switch consists of a common wiper contact, 'C', which can touch one of the other contacts, 1, 2 or 3, at a time. The switch is shown in position 1.

I think you're ON-OFF-ON requirement can be satisfied as shown on the right using only one half of the switch.



After details of switch added:







simulate this circuit

Figure 2. Does this arrangement using one pole of the switch satisfy your requirments?

Table 1. Truth table assuming pullup resistors on GPIO.

Posn   1     2
  1    Low   High
  2    High  High
  3    High  Low