This volume contains data on the geometric parameters (internuclear distances, bond angles, dihedral angles of internal rotation etc.) of free polyatomic molecules including free radicals and molecular ions. This survey on structure data is made of four subvolume, each containing the geometric parameters determined in the gas phase either by an analysis of the rotational constants (and sometimes the vibrational constants) obtained from microwave, infrared, Raman, electronic and photoelectron spectroscopy or by an analysis of electron diffraction intensities. Most of the structures listed in the tables are for molecules in the electronic ground state, but structures for electronically excited states have also been included as far as they are available. In this third subvolume, the structure of molecules containing three for four carbon atoms are listed.