  Resonant three-wave coupling is an important mechanism via which waves
interact in a nonlinear medium. When the medium is a magnetized warm-fluid
plasma, a previously-unknown formula for the coupling coefficients is derived
by solving the fluid-Maxwell's equations to second order using multiscale
perturbative expansions. The formula is not only general but also evaluable,
whereby numerical values of the coupling coefficient can be determined for any
three resonantly interacting waves propagating at arbitrary angles. As one
example, coupling coefficient governing laser scattering is evaluated. In
conditions relevant to magnetized inertial confinement fusion experiments,
lasers scatter from magnetized plasma waves and the growth rates are modified
at oblique angles. As another example, coupling coefficient between two
Alfv\'en waves via a sound wave is evaluated. In conditions relevant to solar
corona, the decay of a parallel Alfv\'en wave only slightly prefers exact
backward geometry.
