First off, technically you can do your 3-way pickup switch with just a
SPDT ON-ON-ON switch, but a
DPDT won't hurt.
An alternative is to use two SPST (or SPDT, or DPDT -- whatever is cheapest or easiest to find) ON/OFF switches -- one per pickup. So that would be 3 switches: 2 for pickups, 1 for passive, but the three swiches would be readily available/cheaper versions, and would give you a mute (with both pickup switches turned off).
Regarding the stereo jack thing, that is the historical way to power
off the preamp when the jack is removed. The mono plug shorts the ring and sleeve on the stereo jack completing the ground for the preamp when plugged in.
An alternative, if you think you can remember to do it, is to use a DPDT switch for the active/passive switch, and have the switch also power off the preamp when you move it to passive mode. A small benefit is that you can stop the drain on your battery while still leaving your patch cable plugged in (if that is your thing). It also
should be a bit quieter when you plug in the bass if your amp is turned on since the tip of the patch cable won't contact the preamp ground on the way into the jack. Also, you can then just use a standard mono jack if that matters to you.
Anyway, wiring isn't that hard -- you just need to think like an electron
Hopefully I didn't just add a bunch more confusion, but if so feel free to ask for clarification. Once you have decided how you want things to work I can give you some better indications of how to actually wire it up.