Top view of a Lego “system” construction of an equilateral triangle including a 5-7-8 inner connection.
Note that if you are counting the studs it’s +1 from 5-7-8. A total of 6 studs is 5 studs center-to-center (which is the distance that matters for the math of the triangle construction, i.e., the centerlines of the elements).
Not shown, but also works: 3-7-8. If you move the bottom attachment point between the blue and the white base two studs to the right it fits (and the math still works exactly).
neilwebber 
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