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Of course Joyoti and Thampy are correct. The reason why the tuning forks are there in the first place, is because the company was started by Mr.Yamaha who used to manufacture and sell musical instruments. The first organ was hauled over a mountain tied on a cart pulled by Yamaha himself. One of its type is in the Museum in Hamammatsu. The three tuning forks and the cart-wheel represent the Yamaha Logo.
The Yamaha Motors came later.... The logo looks the same, but there is a small difference. If you can spot it, let me know vide msg or comment under this. It is subtle, but interesting.