\documentclass[%
,% 
]{scrartcl}

\usepackage{amsmath}

\begin{document}
\begin{align}
        a & \overset{\hphantom{(3.15)}}{=} b + c \\
        b & \overset{(3.15)}{=} 3\cdot c + 2 \\
        c & \overset{\hphantom{(3.15)}}{=} 3
\end{align}
Zum Vergleich ohne \emph{\textbackslash hpanthom{}}:\\
\begin{align}
        a & = b + c \\
        b & \overset{(3.15)}{=} 3\cdot c + 2 \\
        c & = 3
\end{align}

\end{document}