  Pioneering studies in transition metal dichalcogenides have demonstrated
convincingly the co-existence of multiple angular momentum degrees of freedom
-- of spin (1/2 $s_z = \pm 1/2$), valley ($\tau = K, K'$ or $\pm 1$), and
atomic orbital ($l_z = \pm 2$) origins -- in the valence band with strong
interlocking among them, which results in noise-resilient pseudospin states
ideal for spintronic type applications. With field modulation a powerful,
universal means in physics studies and applications, this work develops, from
bare models in the context of complicated band structure, a general effective
theory of field-modulated spin-valley-orbital pseudospin physics that is able
to describe both intra- and inter- valley dynamics. Based on the theory, it
predicts and discusses the linear response of a pseudospin to external fields
of arbitrary orientations. Paradigm field configurations are identified for
pseudospin control including pseudospin flipping. For a nontrivial example, it
presents a spin-valley-orbital quantum computing proposal, where the theory is
applied to address all-electrical, simultaneous control of $s_z$, $\tau$, and
$l_z$ for qubit manipulation. It demonstrates the viability of such control
with static field effects and an additional dynamic electric field. An
optimized qubit manipulation time ~ O(ns) is given.
