The Japanese word for a diamond shape (as opposed to the precious stone that gave the shape its English name) is hishi. This originally meant the water chestnut plant; it came to be used for the rhombus, or diamond shape, because of the shape of the plant's leaves.
In the Japanese language, when the letter 'h' appears in the middle of a word, it is often pronounced as a 'b'; hence Mitsubishi (mitsu meaning 'three').
Yataro Iwasaki, the founder of the first Mitsubishi company, chose the three–diamond logo as it suggests both the crest of the Tosa Clan, his first employer – which shows three oak leaves meeting at a central point – and the three stacked rhombuses that appear in his own family crest. You can see illustrations on the Mitsubishi website, which also shows an earlier version of the logo than the one we're familiar with today. This shows a circle at the central point, where the three diamonds meet – suggesting a propeller.
© Haydn Thompson 2021