The 3 foot rule is the minimum distance between like varieties of hops. Five feet is the minimum for different varieties of hops. Assuming you are containing the hop rhizomes from spreading towards a different hop variety, you also have to watch the bines so their lateral growth does not intermingle with adjacent bines.
Magnum stay close to their bine where Cascades like to sprawl out. See pic:
Left to right is Nugget, Magnum and Cascade. 1 rope for each, 3 bines per rope. Notice how the magnum is compact and the Nugget and especially the Cascade are bushing out sideways. This is where the distance between bines comes into play. The Cascade hop bines will easily intermingle with nearby bines and if they are different varieties, you will have a hard time separating them at harvest.