For those times when a binomial square is not enough The Troutbeck School shows you how to build your own trinomial cube of (a + b + c) out of lego. There is a nice long step by step guide on the school site:
http://thetroutbeckschool.com/how-to-build-a-trinomial-cub…/

lego trinomial cube a layer

lego trinomial cube dissected

lego trinomial cube completed