3. 19 gold coins. As at the end, there was 500 gold coins in the treasury, then after tripling the treasury by the third servant, there was 501 gold coins (that is he should take 1 gold coin after tripling). So, before the third servant came, there was 501 : 3 = 167 gold coins in treasury. Since the second servant tripled and took 1 gold coin out of 167 gold coins, then before the second servant came there was (167 + 1) : 3 = 56 gold coins in the treasury. The same way, before the first servant came, there was (56 + 1) : 3 = 19 gold coins.