The correct ones are: ①

Analysis: ① A triangle with three equal sides is a regular triangle (equilateral triangle), which is a definition and definitely correct.

② A quadrilateral with four equal sides is a square, not necessarily a diamond.

③ An equilateral hexagon, each angle is 120, but the six sides are not necessarily equal, so it is not necessarily a regular hexagon.

I hope my answer is helpful to you.