There are 9 pseudo-tricubes with a total volume of 27 cubes.
A 3x3x3 cube can be made form these pieces. This is a nice puzzle
which is available in trade. Three other possible constructions are shown.
Solutions for these figures are here.
If you only use the five flat pieces with a total of 15 cubes you may ask
for a 3x5 rectangle but this isn't possible.
Instead of you can try for some symmetric figures in two or three dimensions