Six bands of the 3A system (c 3 -a 3 ) in the CO molecule have been photographed as an emission spectra by the Geissler tube. The conventional technique of spectroscopy has been implemented. It was the first attempt to analyze the 1-3 band in the 13 C 16 O molecule as well as the 1-2 band in the 12 C 16 O molecule at high resolution. The Th lines were used as a standard along with the interferometric comparator equipped with a photoelectric scanning device. The 0-2 and 0-3 bands in 12 C 16 O and the 0-3 band in 13 C 16 O and 14 C 16 O were reanalyzed. The total number of lines studied within the framework of this project amounts to 3071. Due to complicated and not totally described perturbations which appeared in the upper state c 3 (v = 0, 1), the effective rovibronic structure constants for this state were calculated (for the first time in the case of v = 1 level in the 12 C 16 O). Global merging of the rotational constants of the a 3 (v = 0, 1, 2, 3) and c 3 (v = 0, 1) levels in 13 C 16 O, as well as a 3 (v = 2, 3) and c 3 (v = 0, 1) in 12 C 16 O, made it possible to obtain c 3 vibrational differences ν 1-0 = 1742.0944 ( 22) cm -1 in 13 C 16 O as well as 1775.7665 (41) cm -1 in 12 C 16 O for the first time. The so far unknown band origins of the 0-v and 1-v progressions of the 3A system in the molecules under consideration have also been determined. The first attempt to specify the equilibrium molecular constants for the c 3 state, along with the new values for the a 3 state, on the basis of the obtained values of merged rovibronic constants has been undertaken. The vibrational constants of the c 3 state in the 13 C 16 O molecule as well as in the 12 C 16 O molecule were thus obtained for the first time. The 1-2 band of 12 C 16 O and Dunham's isotopic relationship were implemented for that purpose. The analysis of anomalously small values of the centrifugal distortion constant D of the c 3 state and the global character of perturbations in the observed isotopes of the CO molecule suggest homogeneous interaction between c 3 (v = 0, 1) and the k 3 state. Simultaneously, the analysis of the comparatively large values of the -doubling parameter q and the analysis of the shifts of the term values caused by perturbations presuppose a significant influence of a heterogeneous nature of any of the 3 states (most probably the j 3 + state) on the c 3 (v = 0, 1) state. The article also presents the first attempt to calculate RKR potentials, r -centroids, and Franck-Condon factors for the 3A system.