  The influence of nonequilibrium initial values of the order parameter on its
evolution at a critical point is described using a renormalization group
approach of the field theory. The dynamic critical exponent $\theta'$ of the
short time evolution of a system with an $n$-component order parameter is
calculated within a dynamical dissipative model using the method of
$\varepsilon$-expansion in a three-loop approximation. Numerical values of
$\theta'$ for three-dimensional systems are determined using the Pad\'{e}-Borel
method for the summation of asymptotic series.
