Map of Maryland where William Sherwill/Sherrill Sr. and Adam Sherrill owned land.

Click on a location to see the document for that land from the Maryland State Archives.

It is clear that William Sherrill, Sr. (the Conestoga Trader), Adam and William, Jr. his sons, were accomplished sailors.
You can see the routes they traveled back and forth up and down the Chesapeake Bay to the Elk River, the Susquehanna River and the Potomac River.
Note: I am only guessing at the location of Three Partners because I could find none of the land names or landmarks mentioned in the document (i.e. Abraham Pennington or Cornwallis' Addition).