You don't need a new style. The modification can be done by \DeclareLabelalpha. It is explained in the documentation at page 153ff and the definition is done in biblatex.def.
Related to your needs the following should work:
\documentclass{article}
\usepackage[style=alphabetic]{biblatex}
\addbibresource{biblatex-examples.bib}
\renewcommand*{\labelalphaothers}{}
\renewcommand*{\sortalphaothers}{}
\DeclareLabelalphaTemplate{
\labelelement{
\field[final]{shorthand}
\field{label}
\field[strwidth=3,strside=left,names=1]{labelname}
}
\labelelement{
\field[strwidth=4,strside=right]{year}
}
}
\usepackage{filecontents}
\begin{filecontents}{short.bib}
@book{short,
author = { Lu, LongLong },
title = {Title of book},
year=2013,
}
\end{filecontents}
\addbibresource{short.bib}
\begin{document}
\cite{companion,short}
\printbibliography
\end{document}
