Cut roast into 3 equal pieces.  Place roast in a large pot and fill with water.  Boil roast until very tender, adding water as necessary.  When roast is done, place each piece of roast on a plate and shred with two forks.  Meanwhile, bring any water remaining from the roasts to a boil; you will need 1 1/2 cups of water from the roasts, so add water if you don't have enough.  Pour brown gravy mix into a small bowl and add 1/2 cup cold water.  Mix well to dissolve all of the gray mix.  Add 1 1/2 cups of roast water to the gravy mix and stir.  Add this mixture to the shredded beef.  Serve beef on French bread with lettuce, tomatoes, and mayonnaise.