PUZZLEBOXX

To find the centre of the circle: fold it in half and make a crease - fold it in half again and make a second crease - where the creases cross is the centre.

Fold the circle so that a point on the circumference exactly touches the centre point (the red flap in the diagram).  Now fold again so that the circumference touches the centre - but also to form a corner where the two flaps meet (the yellow flap in the diagram).  When you fold the third flap (the blue one) to the centre, it should neatly form two corners with the previous folded flaps

You now have an equilateral triangle.  Measure the sides to check.

You could tuck the blue flap partly under the yellow flap for neatness.