You have to give us more details such as the type of pickup switch (whether Gibson style toggle switch or Strat type blade switch) and volume/tone configurations (4 knob Vol-Tone Vol-Tone) on the present HB x HB set up.
What do you think about 2 blend pots? The first one to blend in the Middle Strat PU, the second to blend the bridge PU from singlecoil to humbucker. The 3 way switch will work normally neck/neck+bridge/bridge. The two other pots wil be master volume and master tone.
Just hook up the guitar like the schematic I have linked you to a previous thread in this forum. Each pickup can be blended into any mix by increasing or lowering its volume.
For example :
1. With the toggle switch on neck pickup, roll up the middle volume to blend in for single coil "quack".
2. With the toggle switch in the middle position, roll up the middle to have all three pickups on.
3. With the toggle switch on the bridge pickup, roll up the middle volume to blend in with the humbucker or single coil it it has been split.
4. With the toggle switch in any position, roll off the volumes of the neck and bridge pickups and raise the volume of the middle to have it on.
Use a push-pull pot for the bridge volume control and follow the manufacturer's diagram to split the humbucker.