  We present a combined experimental and theoretical study of valley
populations in the valence bands of trilayer WSe$_2$. Shubnikov$-$de Haas
oscillations show that trilayer holes populate two distinct subbands associated
with the $K$ and $\Gamma$ valleys, with effective masses 0.5$m_e$ and $1.2m_e$,
respectively; $m_e$ is the bare electron mass. At a fixed total hole density,
an applied transverse electric field transfers holes from $\Gamma$ orbitals to
$K$ orbitals. We are able to explain this behavior in terms of the larger layer
polarizability of the $K$ orbital subband.
