  Structural calculations and high-resolution infrared spectra are reported for
trimers and tetramers containing CO$_2$ together with CO and/or N$_2$. Among
the 9 clusters studied here, only (CO$_2$)$_2$-CO was previously observed by
high-resolution spectroscopy. The spectra, which occur in the region of the
$\nu_3$ fundamental of CO$_2$ (~2350 cm$^{-1}$), were recorded using a tunable
optical parametric oscillator source to probe a pulsed supersonic slit jet
expansion. The trimers (CO$_2$)$_2$-CO and (CO$_2$)$_2$-N$_2$ have structures
in which the CO or N$_2$ is aligned along the symmetry axis of a staggered
side-by-side CO$_2$ dimer unit. The observation of two fundamental bands for
(CO$_2$)$_2$-CO and (CO$_2$)$_2$-N$_2$ shows that this CO$_2$ dimer unit is
non-planar, unlike (CO$_2$)$_2$ itself. For the trimers CO$_2$-(CO)$_2$ and
CO$_2$-(N$_2$)$_2$, the CO or N$_2$ monomers occupy equivalent positions in the
'equatorial plane' of the CO$_2$, pointing toward its C atom. To form the
tetramers CO$_2$-(CO)$_3$ and CO$_2$-(N$_2$)$_3$, a third CO or N$_2$ monomer
is then added off to the 'side' of the first two. In the mixed tetramers
CO$_2$-(CO)$_2$-N$_2$ and CO$_2$-CO-(N$_2$)$_2$, this 'side' position is taken
by N2 and not CO. In addition to the fundamental bands, combination bands are
also observed for (CO$_2$)$_2$-CO, CO$_2$-(CO)$_2$, and CO$_2$-(N$_2$)$_2$,
yielding some information about their low-frequency intermolecular vibrations.
