3 is a pentagon - it has five sides.
7 is a quadrilateral - it has four sides.
I cannot see what number 4 is, as you have cut it off with the picture.
3 is a irregular pentagon (pentagon, penta=five, regular means all equal sides and angles)
4 is, I cant see the bottom, if there are only 3 lines, then the figure is a triangle
if ther are 4 sides then it is either, (if the sides are not parrelel then the figure is a quadrilateral) (if 2 sides are parrelell then it is a trapizoid or trapezium)
7 is a quadrilateral because it has 4 sides and cannot be classified further because it doesnt have parralell sides