If this is what is meant by 3 year rule, then in my opinion, this is utter nonsense. Having the 3 youngins play and bond together at a similar age is why we deliberately had them as close as possible. Its great to see the bonds develop.
My sister and I are three years apart and we definitely always had a strong bond. I can’t imagine a year or two would have made a difference. Our other siblings are as far as decades apart from us and we are less close (still close though), but we also didn’t grow up together - my sister and I had already flown the coop. I’d guess there are some limits in between there, but it’s just a guess.