1.Besides the items (like the hawk, shield, bow, etc), can i also drop Gold in any place of the map (for exemple, drop 2 golds at the market so that in a future play when a friend is in the market he could take those 2 gold to buy something more useful to him?)
Yes you can!
2.Regarding combat, the instructions says that when enemies roll dices with the same numbers they sum those points Do this apply to all enemies (in the Portuguese rules it first implies that is does apply to all enemies but further on in states just the Trolls)
Yes to all enemies, even (or especially) to wardraks and other guys with big black dice.
3.Goatskin - Can i use the Goatskin on a friend turn?
From what i have understand we can only use the goatskin when we decide to move our character on our turn. We can also just move with the goatskin, therefore not wasting any hours but cant do anything more then.
You can use Goatskin only if you chose action "movement" in your turn.