Neck and bridge in a standard HH setup are parallel with each other. In a HHH setup, stock anyways, the 3-way toggle is neck, middle+bridge, bridge. There isn't a neck+bridge option without modifying the wiring, usually by giving the middle it's own dedicated volume by sacrificing one of the tone controls.
this.
even on a two pup lp or sg or any other factory wired gibson (other than the jp model or other outliers) the pups them selves are wired in series (normal humbucker wiring), when both pups are on the two series wired humbuckers are in parallel, just like on a typically wired fender guitar. the neck/middle or middle/bridge positions are parallel, not series