  Two-dimensional (2D) transition metal dichalcogenides (TMDC) and their
moir\'e interfaces have been demonstrated for correlated electron states,
including Mott insulators and electron/hole crystals commensurate with moir\'e
superlattices. Here we present spectroscopic evidences for ordered bosons -
interlayer exciton crystals in a WSe2/MoSe2/WSe2 trilayer, where the enhanced
Coulomb interactions over those in heterobilayers have been predicted to result
in exciton ordering. While the dipolar interlayer excitons in the heterobilayer
may be ordered by the periodic moir\'e traps, their mutual repulsion results in
de-trapping at exciton density n_ex larger than 10^11 cm^-2 to form mobile
exciton gases and further to electron-hole plasmas, both accompanied by
broadening in photoluminescence (PL) peaks and large increases in mobility. In
contrast, ordered interlayer excitons in the trilayer are characterized by
negligible mobility and by sharper PL peaks persisting to n_ex approximately
10^12 cm^-2. We present evidences for the predicted quadrupolar exciton crystal
and its transitions to dipolar excitons either with increasing n_ex or by an
applied electric field. These ordered interlayer excitons may serve as models
for the exploration of quantum phase transitions and quantum coherent
phenomena.
