  The three-loop effective potential of the massless O(N) $\phi^4$ theory is
calculated analytically using techniques of dimensional regularization. We see
a complete agreement between our result and Jackiw's result obtained only up to
two-loop order using a different regularization (cutoff regularization) method,
but the same renormalization conditions. For an easy check of the mutual
cancellation of all the dangerous pole terms in each loop order, we give the
$\epsilon$-expanded loop integrals in full detail.
