Align within align

You can use flalign; with \lefteqn the width of the argument is ignored.

\documentclass{article}
\usepackage{amsmath}

\begin{document}

Some text before the display just to see where the margins of the type block
are, some more to be sure
\begin{flalign*}
(1)\quad A &\to \lefteqn{(*some terms*)} &&& \\
& & A.a &= (*some terms*) \\
& & A.b &= (*some terms*) \\
& & A.c &= (*some terms*) \\
(2)\quad B &\to \lefteqn{(*some terms*)} \\
& & B.a &= (*some terms*) \\
& & B.b &= (*some terms*) \\
& & B.c &= (*some terms*) \\
(3)\quad C &\to \lefteqn{(*some terms*)} \\
& & C.a &= (*some terms*) \\
& & C.b &= (*some terms*) \\
& & C.c &= (*some terms*)
\end{flalign*}
Some text before the display just to see where the margins of the type block
are, some more to be sure

\end{document}

You can always use the capabilities of eqparbox to provide similarly-sized widths across document elements (like enumerate or align):

\documentclass{article}

\usepackage{amsmath,eqparbox,enumitem}

\begin{document}

Some text before the display just to see where the margins of the type block
are, some more to be sure:
\begin{enumerate}[label={(\arabic*)}]
  \item
  $\eqmakebox[LHSitem][r]{$A$} \to text{(*some terms*)}$
  \begin{align*}
    \eqmakebox[LHS][r]{$A.a$} &= \text{(*some terms*)} \\
    \eqmakebox[LHS][r]{$A.b$} &= \text{(*some terms*)} \\
    \eqmakebox[LHS][r]{$A.c$} &= \text{(*some terms*)}
  \end{align*}
  
  \item
  $\eqmakebox[LHSitem][r]{$AB$} \to \text{(*some terms*)}$
  \begin{align*}
    \eqmakebox[LHS][r]{$AB.a$} &= \text{(*some terms*)} \\
    \eqmakebox[LHS][r]{$AB.b$} &= \text{(*some terms*)} \\
    \eqmakebox[LHS][r]{$AB.c$} &= \text{(*some terms*)}
  \end{align*}
  
  \item
  $\eqmakebox[LHSitem][r]{$ABC$} \to \text{(*some terms*)}$
  \begin{align*}
    \eqmakebox[LHS][r]{$ABC.a$} &= \text{(*some terms*)} \\
    \eqmakebox[LHS][r]{$ABC.b$} &= \text{(*some terms*)} \\
    \eqmakebox[LHS][r]{$ABC.c$} &= \text{(*some terms*)}
  \end{align*}
\end{enumerate}
Some text before the display just to see where the margins of the type block
are, some more to be sure.

\end{document}