  We introduce a model of a lossy second-harmonic-generating (chi^2) cavity
externally pumped at the third harmonic, which gives rise to driving terms of a
new type, corresponding to a cross-parametric gain. The equation for the
fundamental-frequency (FF) wave may also contain a quadratic self-driving term,
which is generated by the cubic nonlinearity of the medium. Unlike previously
studied phase-matched models of chi^2 cavities driven at the second harmonic
(SH) or at FF, the present one admits an exact analytical solution for the
soliton, at a special value of the gain parameter. Two families of solitons are
found in a numerical form, and their stability area is identified through
numerical computation of the perturbation eigenvalues (stability of the zero
solution, which is a necessary condition for the soliton's stability, is
investigated in an analytical form). One family is a continuation of the
special analytical solution. At given values of parameters, one soliton is
stable and the other one is not; they swap their stability at a critical value
of the mismatch parameter. The stability of the solitons is also verified in
direct simulations, which demonstrate that the unstable pulse rearranges itself
into the stable one, or into a delocalized state, or decays to zero. A soliton
which was given an initial boost C starts to move but quickly comes to a halt,
if the boost is smaller than a critical value C_cr. If C>C_cr, the boost
destroys the soliton (sometimes, through splitting into two secondary pulses).
Interactions between initially separated solitons are investigated too. It is
concluded that stable solitons always merge into a single one. In the system
with weak loss, it appears in a vibrating form, slowly relaxing to the static
shape. With stronger loss, the final soliton emerges in the stationary form.
