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http://www.mitsubishi.com/e/group/mark.html
http://en.wikipedia.org/wiki/Mitsubishi_Group

The three-diamond Mitsubishi mark embodies over 130 years of tradition [...].

The name "Mitsubishi" refers to the three-diamond emblem. "Mitsubishi" is a combination of the words mitsu and hishi. Mitsu means three.
Hishi means water chestnut, and Japanese have used the word for a long time to denote a rhombus or diamond shape. Japanese often bend the "h" sound to a "b" sound when it occurs in the middle of a word. So they pronounce the combination of mitsu and hishi as mitsubishi.
Yataro Iwasaki, the founder of the old Mitsubishi organization, chose the three-diamond mark as the emblem for his company. The mark is suggestive of the three-leaf crest of the Tosa Clan, Yataro's first employer, and also of the three stacked rhombuses of the Iwasaki family crest.