When the Tripods Came by John Christopher (Good, 1999, Pbk, 151 pgs, Simon & Schuster)

When it comes to alien invasions, bad things come in threes. Three landings. One in England, one in Russia, and one in the United States. Three long legs, crushing everything in their paths, with three metallic arms, snaking out to embrace -- and then discard -- their helpless victims. Three evil beings, called Tripods, which will change life on this planet forever.