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\newenvironment{myenv}[1][2cm]{\begin{adjustwidth}{#1}{}}{\end{adjustwidth}}
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\documentclass{article}
\usepackage{lipsum}% http://ctan.org/pkg/lipsum
\begin{document}
\lipsum[1]
\hfill\begin{minipage}{\dimexpr\textwidth-3cm}
\lipsum[2]
\xdef\tpd{\the\prevdepth}
\end{minipage}
\prevdepth\tpd\lipsum[3]
\end{document}
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