http://www.mitsubishi.or.jp/e/h/origin.htmlThe 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.
Three Tosa leaves join three stacked rhombuses to become three red jewels.
A perennial subject for speculation is why Yataro chose not to put his own family name on his new company. If he had, we now might speak of Iwasaki Heavy Industries, Iwasaki Electric, and Bank of Tokyo-Iwasaki. But he opted for a different name.
Whatever Yataro's reason for refraining from using the Iwasaki name, it surely wasn't modesty. Yataro was a bold and aggressive leader. Still, he might have felt some deference toward the Tosa Clan. Support from the clan was instrumental in launching his inaugural venture in shipping. And putting his own name on that venture might have seemed overly impudent, even to Yataro Iwasaki.