They shall make Me a Sanctuary, and I will dwell among them (Ex. 25:8)
Three different types of metal were used to build the Sanctuary: gold, silver and copper. As gold is obviously the most precious, wouldn't it have been more appropriate to use only gold? Rather, the three metals allude to the three categories of Jews. Silver (kesef) alludes to tzadikim (the righteous), who continually yearn (nichsafim - from the same root as kesef) for G-d and His Torah. Gold (zahav) alludes to penitents, "in whose place even complete tzadikim cannot stand." Copper (nechoshet) alludes to those who have transgressed, who have yielded to the temptation of the nachash (serpent) that brought sin into the world. Because G-d wanted all Jews to participate in the Sanctuary's construction, all three metals were utilized. (Maayan Chai)
Reprinted from Parshat Terumah 5762/2002 edition of L’Chaim