I just looked at that blog - the number in the union of the three sets should be identical to the number who are in 'at least one set', so there's some kind of error there.

In any case, I've always found 3-overlapping set formulas both difficult to remember and awkward to apply. I would never even consider using them on a GMAT problem, and I certainly don't think you need to know them, provided you're comfortable working your way through a Venn diagram. The GMAT really is not a test of whether you are a human spreadsheet program - that is, it's not testing whether you can memorize formulas and plug numbers into them. Most GMAT questions include some kind of conceptual 'twist' so that the question is testing your ability to actually *think* through a math problem. Because of those twists, you often won't be able to simply plug numbers into some formula you've memorized to get your answer. That's one reason I much prefer a Venn diagram approach to overlapping set problems - then you aren't relying on seeing questions which ask the precise question these formulas answer.