Align centered, ragged right and ragged left in align environment

You did say everything right of the equal sign should be ragged left. I don't think you meant it, but I show it as a possibility.

Done here with TABstacks, after showing the OP's original code.

\documentclass{article}
\usepackage{amsmath,tabstackengine}
\TABstackMath
\begin{document}
\begin{align*}
        && a &= b\\
        \Leftrightarrow&& b &= c\\
        \Leftrightarrow&& c + 2 &= 3 + 5\\
        \Leftrightarrow&& d + c + 1 &= 4 + 11 + 12
\end{align*}
\[
\setstackaligngap{6pt}% horizontal gap between align groups
\alignCenterstack{
        && a =& b\\
        \Leftrightarrow&& b =& c\\
        \Leftrightarrow&& c + 2 =& 3 + 5\\
        \Leftrightarrow&& d + c + 1 =& 4 + 11 + 12
}
\]
\[
\setstacktabbedgap{0pt}% horizontal gap between tabbed groups
\tabbedCenterstack[r]{
        & a =& b\\
        \Leftrightarrow& b =& c\\
        \Leftrightarrow& c + 2 =& 3 + 5\\
        \Leftrightarrow& d + c + 1 =& 4 + 11 + 12
}
\]
\end{document}

You already have alignat

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{alignat*}{2}
        &\;& a &= b\\
        \Leftrightarrow && b &= c\\
        \Leftrightarrow && c + 2 &= 3 + 5\\
        \Leftrightarrow && d + c + 1 &= 4 + 11 + 12
\end{alignat*}
\end{document}