Generally speaking, when an ability is worded in the "Do X to do Y" format, the "X" is a cost. Old Forest Hunter is worded this way ("Discard a card to gain 1 gold), so discarding the card ("X") is the cost of the ability. Paying for one ability cannot be applied to the costs for other abilities, too -- even if the other abilities are on separate copies of the same card.
Discarding one card and getting 3 gold - one for each copy of Old Forest Hunter - would be like going into you local game store, paying the $39.95 cost for a Core Set once and walking out with 3 copies.