  Energy levels and radiative rates (A-values) for four types of transitions
(E1, E2, M1, and M2) are reported for an astrophysically important Mg-like ion
Si~III, whose emission lines have been observed in a variety of plasmas. For
the calculations, well-known and widely-used GRASP code has been adopted, and
results are listed for transitions among the 141 levels of the 3$\ell3\ell'$
and 3$\ell$4$\ell$ configurations. Experimental energies are available for only
the lowest 58 levels but there is no major discrepancy with theoretical
results. Similarly, the A-values and lifetimes show a satisfactory agreement
with other available results, particularly for strong E1 transitions. Collision
strengths are also calculated, with the DARC code, and listed for resonance
transitions over a wide energy range, up to 30~Ryd. No similar results are
available in the literature for comparisons. However, comparisons are made with
the more important parameter, effective collision strength ($\Upsilon$), for
which recent $R$-matrix results are available for a wide range of transitions,
and over a large range of temperatures. To determine $\Upsilon$, resonances
have been resolved in a narrow energy mesh, although these are not observed to
be as important as for other ions. Unfortunately, large discrepancies in
$\Upsilon$ values are noted for about half the transitions. The differences
increase with increasing temperature and worsen as the upper level J increases.
In most cases the earlier results are overestimated, by up to (almost) two
orders of magnitude, and this conclusion is consistent with the one observed
earlier for Be-like ions.
