All three of these metaphors contribute to the tone of the poem, which is dark and foreboding.
The wind was a torrent of darkness: Although the wind helps to establish the setting, creating a harsh opening from the first line, it also should be considered symbolically. Wind often connotes a time of change, which both the highwayman and Bess experience before the poem's end. A "torrent" is most often used to describe a fast-moving body of water that is wild and reckless. The deaths of the highwayman and Bess are unexpected and violent. Their lives are drowned in powers of darkness.
The moon was a ghostly galleon: A galleon was a term used for a sailing ship in the fifteenth through seventeenth centuries; it was first used as a warship before transforming to trading purposes. A "ghostly galleon" conjures images of death as well as war. Sailors often use the moon to determine their position on the open ocean. Eventually, Bess "shatter[s] her breast in the moonlight" as she warns the highwayman of the red coats who hold her captive. The highwayman had relied on the moonlight to bring him back to Bess, promising, "I’ll come to thee by moonlight." As it turns out, the moonlight reflects the death of the couple. It leads them to their deaths instead of reunification.
The road was a ribbon of moonlight: The road between the lovers is not a straight and direct path. Instead, it is complicated with many twists and turns. This can perhaps be seen as a metaphor for the relationship between the highwayman and Bess. They traverse difficult territory to be together, facing danger together even as they travel separately. They rely on the moonlight, which is a "ghostly galleon," to lead them back to each other.