How do put \overbrace across an (aligned) equals sign?

Not quite elegant but works

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{flalign}
  \rlap{$\displaystyle\overbrace{\phantom{f = \sum_{i\in I} a_i}}^{\text{ my notes }}$}
  f &= \sum_{i\in I} a_i  \\
  g &= blabla \\
  h &= blabla
\end{flalign}
\end{document}

Basically you put the expression with the \overbrace in a box and print it first. Using \rlap the expression is printed without taking any horizontal place. Then you print the expression, this time with the alignment &.

The same result can be obtained with \mathrlap from the mathtools package

\documentclass{article}
\usepackage{mathtools} % loads also amsmath
\begin{document}
\begin{flalign}
  \mathrlap{\overbrace{\phantom{f = \sum_{i\in I} a_i}}^{\text{ my notes }}}
  f &= \sum_{i\in I} a_i  \\
  g &= blabla \\
  h &= blabla
\end{flalign}
\end{document}

After 20 minutes I have created this MWE, more ugly ( :-) ) than the answer of @campa ...but also it works.

\documentclass[a4paper,12pt]{article}
\usepackage{mathtools}


\begin{document}
\[
      \begin{matrix}
      \mathclap{\overbrace{f = \displaystyle \sum_{i\in I} a_i  }^{\text{my notes}}} \\[2ex]
          g= \phantom{\displaystyle \sum_{i\in I} a_i} \hspace{-3.5\tabcolsep}\text{abc}  \\
          h=\phantom{\displaystyle \sum_{i\in I} a_i} \hspace{-3.5\tabcolsep}\text{abc}  
      \end{matrix}
      \]
\end{document}