Which is the correct order of C--O bond length in $\mathrm{CO}$, $\mathrm{CO}{}_2$ and $\mathrm{CO}{}_3{}^{2-}$?

Bond order in $\mathrm{CO}$ is 3 (about 113 pm), in $\mathrm{CO}{}_2$ it is 2 (about 116 pm) and in $\mathrm{CO}{}_3{}^{2-}$ it is $4/3$ (about 129 pm). A higher bond order gives a shorter bond, so the order of length is $\mathrm{CO}<\mathrm{CO}{}_2<\mathrm{CO}{}_3{}^{2-}$.