\documentclass{article}
\usepackage[fleqn]{nccmath}

\begin{document}

\begin{align}
a &= b \\
&= c \\
&= d.
\end{align}

\begin{ceqn}
\begin{align}
a &= b \\
&= c \\
&= d.
\end{align}
\end{ceqn}

\begin{equation}
a=b
\end{equation}

\[a=b\]

\end{document}