  We measure the ratio $\gamma$ of the momentum-transfer to the vibrational
quenching cross section for the X ($^1\Sigma^+$), $\nu=1$, $\mathrm{J=0}$ state
of molecular thorium monoxide (ThO) in collisions with atomic $^3$He between
800 mK and 2.4 K. We observe indirect evidence for ThO--He van der Waals'
complex formation, which has been predicted by theory. We determine the 3-body
recombination rate constant $\Gamma_3$ at 2.4 K, and establish that the binding
energy E$_b >$ 4 K.
