I'm not in front of anything I can use to draw up a schematic, but see if this makes sense.
A 3-way switch, Fender / Oak Grisby type, has two sets of four contacts. I'll call one set of four the left (Lfor short) and the other the right (R). Each set of four contacts has a common (COM) lug and three other.lugs that make contact in turn when switched between position 1 (usually the bridge pickup) and position 3 (usually the neck.
Now here's how to wire the 3-way to get what you want.
The neck single coil hot (white or yellow) goes to the P3 lug on one side of the switch. The neck ground (usually black) goes to ground in the back of your volume pot casing.
The bridge humbucker hot (black) goes to the P1 lug on the same side of the switch as the neck hot connection. Add a jumper wire so the bridge hot is also connected to the P2 lug. The bridge humbucker ground (green) and the baseplate / shield wire (bare) go to ground on the back of the volume pot. Red and white get discussed below.
Connect the Common lug on the side where all your pickup hots are connected to your volume pot "in".
Now for the humbucker middle or red and white wires. Connect both of those to the P2 lug on the other side of the switch. Connect the common lug on that side of the switch to.ground.
In P1 the bridge humbucker is connected with both coils active and in series, by virtue of the red and white being connected to each other but not to ground. The neck single coil hot is not connected.
In P2 the humbucker black is connected by virtue of the jumper, but the green, red, and white are all grounded. That leaves the south coil grounded at both ends so it is ineffective. The north coil is grounded by white being connected to ground and the black hot means it's active.
In P3 only the neck single coil is connected.
Hope this helps.
