The origins of Mt. Fugendake, Mt. Kunimidake, and Mt. Myokendake are different although these mountains look like aligning 3 peaks.
Mt. Kunimidake and Mt. Myokendake are a part of topography of slope failure occured about 20,000 year ago during the massive collapse of Myoken Volcano which started volcanic activities about 40,000 years ago.
Mt. Fugendake is one of the lava domes which erupted to fill the large depression generated by the collapse of Myoken Volcano and it was formed about 6,000 years ago.
There is a folk tale of "3 boys of mountain" to compare these 3 mountains as 3 boys of "Kappa" (a japanese monster), and these mountains have been very familiar to local people. However, while Mt. Kunimidake and Mt. Myokendake are like "brothers", Mt. Fugendake is much younger than them.