The magnetic, structural and optical properties of CoX2O4 (X = Cr, Mn and Fe) spinels are calculated using GGA + U approximation. The effect of the octahedral cation X on the properties of these spinels are analyzed. In order to better understand the electronic aspect of these compounds we studied the issue between the relative forces of the exchange effect and the crystal field effect through a complete analysis of the densities of electronic states. Obtaining the correct ground state is only possible if the electron-electron interactions between magnetic cations are introduced. When the X cations are changed, the crystalline structure changes totally from cubic normal spinel for CoFe2O4 to tetragonal normal spinel one for CoMn2O4 to inverse spinel for CoCr2O4. The electronic properties of our spinels are significantly different, an increase in the band gap from Fe to Mn to Cr compounds is obtained. Magnetic exchange interactions are strongly affected by sub-lattices occupation in the inverse phase of CoFe2O4 and significant structural distortion of the CoMn2O4 compound. The analysis of structural parameters and electronic structures plays a role on the trends of magnetic exchange interactions. We have noticed that the iron states in CoFe2O4 are extremely localised making this spinel very different from the X cation states in the other two spinels. So the variation in X cations allows us to confirm the trend in the properties of CoX2O4. The prediction of optical properties is possible and it allowed us to calculate different optical parameters. We have noticed that epsilon(1)(0) decreases with increasing band gap.