\documentclass[a4paper,11pt]{scrartcl}

\usepackage[fleqn]{amsmath}

\begin{document}

\begin{flalign}
a_{foo} & = 1,2,3 \\
b_{bar} & = 4,5,6 \nonumber \\
c       & = 7 
\end{flalign} 

\end{document} 