Further thoughts. If you wish to just add the coil tap feature to the third pickup, you could use a 3PDT switch instead of the DPDT you mentioned. However, I am not aware of a 3PDT switch combined with a pot.
The DPDT switch board offered here is designed to add components to be switched. The 3PDT foot switch board would not work because it has an internal trace that connects poles 3 and 9 that is used for signal bypass.