Quantitative imaging of chemical composition may be accomplished using dual energy CT. However, dual energy CT can only quantify at most two materials because dual energy CT provides two independent measures with the dual (two) energy x-ray spectra. However, it is very common to need to quantify more than two materials. We have developed a technique to quantify three materials that is an extension of traditional dual energy two material composition methods. The main advantage, and inventive step, of this method is that we accurately quantify three materials without acquiring any additional data. This technique can be used in general quantitative imaging in which three materials are considered.