Everybodys taking this WAY too seriously, I don't believe it for a second but I got to admit the similarities here, nice find- I would've never noticed that otherwise. +rep
It's a very interesting and believable, not likely however, theory. I love the idea of it! Perhaps the 3 Sannin weren't one of the steps in the ino-shika-cho formation, but actually the inspiration for it. The three may be from those clans but had no pressures to follow clan customs. The clans saw that the three clans could work well together with their respective jutsus.