What is it you want? All three pups or bridge and middle? Neck (bass) and middle are pretty common. Bridge (treble) is usually alone. a 5 position switch should give you, neck, neck and middle, middle, middle and treble, treble. I really don't know the tone caps for each position or the resistors and hookup. A standard five position Fender hookup should work. Maybe a Gibson. It'll depend on your PUPs.

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Mike look at this blender pot.

I think you're thinking about a 3 lug blender and that's
why the light isn't on.

This will do exactly what you want if I'm following you.

By running the 3 way switch to control each pup and sending-from the switch
the neck and bridge you will be able to blend either the neck and bridge.
If the switch is up for bridge you can blend the neck in or leave it out. The middle is out.
If the switch is in middle - neck and bridge out- middle only.
If the switch is down for neck- any blend you want for neck and bridge- The middle is out.

One volume and tone for middle. ( Maybe R1  in your schematic)
One volume and tone for neck and bridge. ( Maybe R2 in your schematic)

Blender pot is only connected to neck or bridge at switch.
 
Hope I haven't screwed up. I'm good at that. :)

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Mike what I've suggested is so you can use the same parts you have.

Wire the middle pup to the 3 way switch - It will be nothing more than an on-off for the middle pup. Use one volume and tone for it.

Wire the N and B to the blender and use the other volume and tone for them.

With that you can blend all three pups anyway you want.
 
 
That is IF you have the six lug blender that I gave you a link to...
I asked if you had that or the 3 lug type and you didn't answer.
People can get confused about blender pots. You don't want the 3 lug...it acts
like a bypass.
You want the linear blender- some called it "stacked", "pan" or "ganged"...6 lugs.
That way when it's in the middle (indented) both pups ( neck and bridge) would be 100%.
It  simultaneously turns one up and one down.

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