  We present three-loop formulas for the additive renormalization constant
A(u,epsilon) and associated renormalization group function B(u) for the
specific heat of the O(n) symmetric phi^4 model. Using this result, we obtain
also the amplitude function above Tc within the minimally renormalied theory at
fixed d=3. At the fixed point, the three-loop correction to B(u) turns out to
be small (about 3% for n=2). We note that a correction of this size may become
important at the level of accuracy expected in future experiments.
