Results from the third campaign of a ground-based multiband observation of the millimeter emission of the sky from Tenerife (Canary Islands) are presented. The instrument consists of a 0.45 m diameter off-axis telescope equipped with a four-band multimode 3He cooled photometer working at 1.1, 1.3, 2.1, and 3.1 mm wavelengths. The beam is well approximated by a Gaussian with 1.35d FWHM at all wavelengths. The wide wavelength coverage of our instrument allows us to characterize and reduce both the atmospheric and Galactic contamination in our data. The cosmic microwave background radiation data is analyzed in six multipole bands whose centers span the range l=39 to l=134 at the two longest wavelengths (2.1 and 3.1 mm). A likelihood analysis indicates that we have detected fluctuations in all bands at the two wavelengths. We have evidence of a rise in the angular power spectrum from low l to high l. Our measured spectrum is consistent with current popular theories of large-scale structure formation, COBE, and other recent balloon-borne experiments with similar wavelength coverage.