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Here's a (crap) diagram of why you want the other two fins. Centre only can work, but not so easily.
I've got a surf kayak, and it's got 4 fins on the bottom, plus one each side about ⅓ way up for sharp carving. Without them, it just slides out on bottom turns. I've made adapter plates to take wakeboard fins so I can still spin with 2 of those on the bottom, a couple of longer ones on the sides.
Off topic sorry.... here's your pic.