  The incommensurate stacking of multi-layered two-dimensional materials is a
challenging problem from a theoretical perspective and an intriguing avenue for
manipulating their physical properties. Here we present a multi-scale model to
obtain the mechanical relaxation pattern of twisted trilayer van der Waals
(vdW) heterostructures with two independent twist angles, a generally
incommensurate system without a supercell description. We adopt the
configuration space as a natural description of such incommensurate layered
materials, based on the local environment of atomic positions, bypassing the
need for commensurate approximations. To obtain the relaxation pattern, we
perform energy minimization with respect to the relaxation displacement
vectors. We use a continuum model in combination with the Generalized Stacking
Fault energy to describe the interlayer coupling, obtained from
first-principles calculations based on Density Functional Theory. We show that
the relaxation patterns of twisted trilayer graphene and $\mathrm{WSe_2}$ are
"moir\'e of moir\'e", as a result of the incommensurate coupling two bilayer
moir\'e patterns. We also show that, in contrast to the symmetry-preserving
in-plane relaxation in twisted bilayers, trilayer relaxation can break the
two-fold rotational symmetry about the xy-plane when the two twist angles are
equal.
