These were all intended to be compass and straight edge constructions and all were found to be impossible to do in that way.
The first was to divide a given angle (of no particular size) into three equal angles. It turned out to be impossible even though dividing the angle into two equal parts is very easy.
Duplication of the cube meant finding a way to get a cube double the volume of a given one. You weren't meant to actually make the cube but to construct the length of one edge. Again this was found impossible.
The third problem was to construct a square that had exactly the same area as that of a given circle. Yet again can't be done.
