METALS-London copper drops to three-week low on stronger dollar, China holiday

(Adds detail, updates prices) By Melanie Burton MELBOURNE, May 1 (Reuters) - London copper hit its lowest in three weeks on Tuesday, dampened by a stronger dollar ahead of a U.S. monetary policy meeting and by a holiday in top metals user China. Aluminium and zinc prices also came under pressure after the United States delayed aluminium and steel tariffs for some countries and announced permanent exemptions for others, raising the prospect of better metal supply. FUNDAMENT