'Straight' Parentheses around Matrices

I think you shouldn't mix the parentheses of TX with those of CM, but here's how you can do.

\documentclass{article}
\usepackage{amsmath}
\usepackage{lmodern}

%\usepackage{txfonts} % \lambdaup
\makeatletter
\let\iftx@libertine\iffalse
\let\iftx@minion\iffalse
\let\iftx@coch\iffalse
\makeatother
\DeclareFontEncoding{LMX}{}{}
\DeclareFontSubstitution{LMX}{ntxexx}{m}{n}
\DeclareSymbolFont{TXlettersA}{U}{ntxmia}{m}{it}
\SetSymbolFont{TXlettersA}{bold}{U}{ntxmia}{b}{it}
\DeclareSymbolFont{TXlargesymbols}{LMX}{ntxexx}{m}{n}
\SetSymbolFont{TXlargesymbols}{bold}{LMX}{ntxexx}{b}{n}

\DeclareMathSymbol{\lambdaup}{\mathord}{TXlettersA}{21}
\DeclareMathDelimiter{(}{\mathopen}{operators}{"28}{TXlargesymbols}{0}
\DeclareMathDelimiter{)}{\mathclose}{operators}{"29}{TXlargesymbols}{1}


\begin{document}

$\begin{pmatrix}
1 & 0 & \lambdaup \\
0 & 1 & 0\\
0 & 0 & 1
\end{pmatrix}$

$(a+b)c$

$\bigl(a+b\bigr)c$

\end{document}

If you prefer the parentheses in the style of txfonts, you can do like this:

\documentclass{article}
\usepackage{amsmath}
\usepackage{lmodern}

%\usepackage{txfonts} % \lambdaup
\DeclareSymbolFont{TXlettersA}{U}{txmia}{m}{it}
\SetSymbolFont{TXlettersA}{bold}{U}{txmia}{bx}{it}
\DeclareSymbolFont{TXlargesymbols}{OMX}{txex}{m}{n}
\SetSymbolFont{TXlargesymbols}{bold}{OMX}{txex}{bx}{n}

\DeclareMathSymbol{\lambdaup}{\mathord}{TXlettersA}{21}
\DeclareMathDelimiter{(}{\mathopen}{operators}{40}{TXlargesymbols}{0}
\DeclareMathDelimiter{)}{\mathclose}{operators}{41}{TXlargesymbols}{1}


\begin{document}

$\begin{pmatrix}
1 & 0 & \lambdaup \\
0 & 1 & 0\\
0 & 0 & 1
\end{pmatrix}$

$(a+b)c$

$\bigl(a+b\bigr)c$

\end{document}