  We analytically evaluate the three-loop Feynman integral which was the last
missing ingredient for the analytical evaluation of the three-loop quark static
potential. To evaluate the integral we introduce an auxiliary parameter $y$,
which corresponds to the residual energy in some of the HQET propagators. We
construct a differential system for 109 master integrals depending on $y$ and
fix boundary conditions from the asymptotic behaviour in the limit $y\to
\infty$. The original integral is recovered from the limit $y\to 0$. To solve
these linear differential equations we try to find an $\epsilon$-form of the
differential system. Though this step appears to be, strictly speaking, not
possible, we succeed to find an $\epsilon$-form of all irreducible diagonal
blocks, which is sufficient for solving the differential system in terms of an
$\epsilon$ expansion. We find a solution up to weight six in terms of multiple
polylogarithms and obtain an analytical result for the required three-loop
Feynman integral by taking the limit $y\to 0$. As a by-product, we obtain
analytical results for some Feynman integrals typical for HQET.
