Solution
= Solution
Since $r>3M$,
$$
\boxed{\Delta\tau_1<\Delta\tau_2<\Delta\tau_3}.
$$
Relative to the distant observer, the static clock $O_2$ suffers gravitational <time dilation>. The orbiting clock $O_1$ suffers the same gravitational effect together with the kinematic time dilation caused by its orbital motion, so it accumulates the least proper time.