#3 is a triangular pyramid because triangular pyramids have 4 triangular faces.
#4 is a cube because there are 6 square faces.
#5 is a triangular prism. It has 2 triangular faces and 3 rectangular faces.
#6 is a rectangular prism. It has 2 square faces and 4 rectangular faces.
(If you don't know what the figure of the net is, resketch it on another piece of paper, cut it out, and form it.)
Hope this helps,
Loafly