\begin{enumerate}
\item item 1
\item item 2
\item item 3
\end{enumerate}
How to reduce the space between item 1,2,3?
\begin{enumerate}
\item item 1
\item item 2
\item item 3
\end{enumerate}
How to reduce the space between item 1,2,3?
Comments have provided good answers. Though one can adjust space between items manually without using any package by setting length \itemsep, for instance:
\documentclass{article}
\begin{document}
\noindent\dotfill
\begin{enumerate}
\setlength{\itemsep}{25pt}
\item item 1
\item item 2
\item item 3
\end{enumerate}
\noindent\dotfill
\begin{enumerate}
\item item 1
\item item 2
\item item 3
\end{enumerate}
\noindent\dotfill
\begin{enumerate}
\setlength{\itemsep}{1pt}
\item item 1
\item item 2
\item item 3
\end{enumerate}
\noindent\dotfill
\end{document}
gives:

Other useful lengths include \topsep, \partopsep, \parsep, etc. For more information, check out source2ε.