We report the detection of the 405 nm band of interstellar C3 in absorption toward HD 210121. The abundance of triatomic carbon is approximately 1/17 of that of diatomic carbon in the same diffuse molecular cloud. Rotational levels of C3 up to J=14 are seen in this cloud. The rotational excitation of C3 in the interstellar medium may reflect a competition between inelastic collisions, formation and destruction of the molecule, and radiative pumping in the far-infrared. The abundance of C3 is compared with chemical models. Attention is called to molecular properties that need to be better determined. Based on observations collected at the European Southern Observatory, Paranal, Chile [ESO VLT-UT2 No 65.I-0526(A)].