The authors present optical and near infrared photometry, covering from 0.36 to 2.3 microns, for a wide sample of red giant stars in the globular cluster M3. These measurements are used to make a first attempt in order to apply the Infrared Flux Method to obtain the observational (L - Teff) diagram for cluster stars. The peculiarities of the method for this particular application are discussed. The estimated errors in Teff are ±100K and most of the uncertainty present in the luminosities is due to distance errors. From a comparison with the theoretical evolutionary tracks (Rood, 1972), a variation in the mixing length parameter seems necessary to situate the mean red giant branch in a position in accordance to previous metallicity estimations.