Magic The Gathering, Double damage, Doubled' red cards?

Ok so if i have in play 2"Furnace of Rath"(-if a source would deal damage,that source deals double that damage-) and "Fire Servant"(If a red instant or sorcery would deal damage it deals double that damage-) are all in play and working, would a red instant "Lighting Bolt (deal 3 damage to target creature or player) does the damage get doubled 3 times stacking and become 3 times 2 times 2 times 2 = 24 damage or would it only double the original 3 and make it 3 times 2 plus 3 times 2 plus 3 times 2 = 18 damage? also does Furnace of Rath make combat damage double? it doesn't specify. and if i activate something like Fling (sacrifice a creature deal damage equal to its power) choosing Fire Servant as the sacrifice does Fire Servants effect activate as it is being sacrificed?