\documentclass{minimal}
\usepackage{tikz}
\usetikzlibrary{circuits.logic.US,circuits.logic.IEC}
\begin{document}
\begin{tikzpicture}[minimum height=0.75cm]
\node[or gate US, draw,logic gate inputs=nnn] (A) {};
\foreach \a in {1,...,3}
\draw (A.input \a -| -1,0) -- (A.input \a);
\draw (A.output) -- ([xshift=0.5cm]A.output);
\end{tikzpicture}
\begin{tikzpicture}[minimum height=0.75cm]
\node[and gate US, draw,logic gate inputs=nnn] (A) {};
\foreach \a in {1,...,3}
\draw (A.input \a -| -1,0) -- (A.input \a);
\draw (A.output) -- ([xshift=0.5cm]A.output);
\end{tikzpicture}
\end{document}

By slight modification of this code you can acheive gates with more inputs (modifying inputs=nnn
and \foreach \a in {1,...,3}
).
For more, check TikZ & PGF manual section 29.3 Page 300