In geometry, a 7-chord is a line segment that connects two points on a circle and divides the circle into two parts. A triangle can be formed by connecting the endpoints of a 7-chord with the center of the circle. The relationship between a triangle and a 7-chord is that the 7-chord is a side of the triangle when the center of the circle is one of the triangle's vertices.
They are B flat,D,F and A flat
If you want the progression, E, A, B is right. That is a one, four, five progression, one of the most common blues progressions. You can fit other chords to fill in the key of E. F#min would work as a follow up chord. G#min would follow that if you like. Then A would fit. It depends on the feel. Major chords work too. "Smoke On The Water" is E, G, A, then E, G, A# A, then it repeats. It depends also on the arrangement. You may use E, followed by D, then A. If it sounds like what you were thinking, it's right.