There were 3000 gold coins, 4000 silver and 5000 bronze. Assume each statement to be true in turn and examine the consequences. If Dimwit is telling the truth, there are 3000 silver coins, 5000 bronze coins and hence 4000 gold coins. However, this would make Beefbrain's statement true also. By symmetry if Beefbrain is truthful so is Dimwit. Only if Thickplank is telling the truth are the other statements false.