That wiring diagram is a bit strange.
Your pickup selections on the switch will give you a) bridge B) neck+ bridge and c) neck.
Then the middle pickup is on what would be a tone pot as its volume control. So if that volume control is anywhere but fully off, you are blending it in with the main pickup selection, so you get a) bridge+middle, b) bridge+middle+neck and c) middle+neck with various levels of middle pickup. These will all have the slightly phasey Strat mixed-pickup sound, with b) sounding thinnest of all.
You need to have the main volume off and the middle volume turned up to get just the middle pickup.
So for bridge pickup on its own you need the bridge pickup only selected on the switch, the main volume on and the middle volume off.
For middle pickup on its own, the middle pickup volume is on and the main volume is off (regardless of switch position)
For the neck pickup on its own you need the neck pickup only selected on the switch, the main volume on and the middle volume off.
It all seems overly complex to me and it would be a lot easier to get all the possible pickup combinations with a 5-way switch and a switched pot to always link the neck or bridge pickup to the selector switch output. I know I'd always get it wrong!
If you just wanted bridge, middle or neck pickup only selections, then this is not the way to do it.