Multiple side-by-side signatures in letter

Instead of minipages you can use \parboxes:

\documentclass[%
paper=letter,
pagesize,
fontsize=10pt,
standard
]{scrlttr2}
\usepackage[english]{babel}
\usepackage{lipsum}

\newcommand\Sigline[4][t]{%
  \parbox[#1]{.333\linewidth}{\raggedright#2}%
  \parbox[#1]{.333\linewidth}{\raggedright#3}%
  \parbox[#1]{.333\linewidth}{\raggedright#4}%
}

\setkomavar{fromname}{Sender's Name}
\setkomavar{fromaddress}{Some street\\in some town\\with zip code}
\setkomavar{signature}{% 
   \Sigline{First name}{Second name}{Third name} \\
  \Sigline{Position 1}{Position 2}{Position 3} \\
}

\begin{document}
\begin{letter}{%
Recipient's name,\\
Some position\\
More address info
}

\opening{Dear Recipient}
\lipsum[2]

\closing{Sincerely,}

\end{letter}
\end{document}

Here is a solution that is independent of any particular document class.

\documentclass{minimal}

\begin{document}

Lorem ipsum dolor sit amet, consetetur sadipscing elitr,
sed diam nonumy eirmod tempor invidunt ut labore
et dolore magna aliquyam erat, sed diam voluptua.

\def\s#1#2{\vbox{\hsize=4.5cm
    \kern2cm
    \hrule\kern1ex
    \hbox to \hsize{\strut\hfil #1 \hfil}
    \hbox to \hsize{\strut\hfil #2 \hfil}}}

\hbox to \hsize{\s{John Doe}{President}\hfil
    \s{Jane Doe}{COO}\hfil
    \s{James Dow}{CFO}} 

\end{document}