A new spectrophotometric method was developed for the simultaneous analysis of a ternary mixture, without prior separation. This method is based on the use of the derivative of the ratio spectrum obtained by dividing the absorption spectrum of the ternary mixture by a standard spectrum of a mixture of two of the three compounds in the title mixture. The concentrations of three compounds in their mixture are determined by using their respective calibration graphs which are obtained by measuring the amplitude at either the maximum or minimum wavelengths selected. The mathematical explanation of the procedure is illustrated. The method was applied for the assay of tablets containing metamizol, caffeine and paracetamol. The developed method was compared with alternative spectrophotometric methods.