Increase spacing in split environment

The length \jot determines the space in a multi-line amsmath enironment. You could adjust it, affecting all lines or use the optional argument of \\ to increase the spacing specifically.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
\setlength{\jot}{10pt} % affecting the line spacing in the environment
\begin{split}
1
\\
2
\\[3ex]% additional space affecting just this line
3
\end{split}
\end{equation}
\end{document}

Changing \jot inside the outer equation environment limits the scope of this action.

But as it seems that you just want to adjust certain lines, using \\ at this point should be sufficient.

The traditional way is

\begin{equation}
\openup 1\jot
\begin{split}
...
\\
...
\\
...
\end{split}
\end{equation}

Edit: From the definition in ltmath.dtx (which is copied directly from plain tex)

\def\openup{\afterassignment\@penup\dimen@}
\def\@penup{\advance\lineskip\dimen@
   \advance\baselineskip\dimen@
   \advance\lineskiplimit\dimen@}

Ie, \openup dimen just increases the line spacing by dimen. Traditionally, dimen is given in units of \jot, which often set to 3 pt. It's all described in the TeXBook.