Make colsep part of the column width

[al3atross]

Problem

I'd like to have a table with column ratio of 3:1.

X columns seem to be the right approach as they occupy the remaining width in proportion to their coefficients.

Unfortunately, the colsep dimension is not part of the column width.
That's why it is subtracted from the remaining width before the column widths are determined.

This results in different positions of the Vline depending on the colsep as the following example illustrates.

\documentclass{article}
\usepackage{tabularray}

\begin{document}

\begin{tblr}{colspec={|X[3]|X[1]|}, hlines, colsep=0pt}
	A & B \\
\end{tblr}

\vspace{3mm}
	
\begin{tblr}{colspec={|X[3]|X[1]|}}
	A & B \\
\end{tblr}

\vspace{3mm}

\begin{tblr}{colspec={|X[3]|X[1]|}, hlines, colsep=24pt}
	A & B \\
\end{tblr}

\end{document}

While the first table (with colsep=0pt) has a Vline-to-Vline ratio of 3:1, the last table (with colsep=24pt) has a Vline-to-Vline ratio of about 2.3:1.

Proposal

My proposal is to include the 'colsep' into the column width such that the Vline-to-Vline ratio is maintained.
(There might be other dimensions like rulewidth that have to be taken into account to achieve the exact ratio.)

[lvjr]

As I have commented below another post of yours, you should not use X columns if you need fixed-width ciolumns:

\documentclass{article}
\usepackage{tabularray}
\usepackage{calc}
\begin{document}

\newlength\lengtha \setlength\lengtha{0.75\linewidth-0.4pt}
\newlength\lengthb \setlength\lengthb{0.25\linewidth-0.4pt}

\begin{tblr}{colspec={|Q[\lengtha]|Q[\lengthb]|}, hlines, colsep=0pt}
	A & B \\
\end{tblr}

\vspace{3mm}

\newlength\lengthc \setlength\lengthc{0.75\linewidth-0.4pt-12pt}
\newlength\lengthd \setlength\lengthd{0.25\linewidth-0.4pt-12pt}

\begin{tblr}{colspec={|Q[\lengthc]|Q[\lengthd]|}, hlines}
	A & B \\
\end{tblr}

\vspace{3mm}

\newlength\lengthe \setlength\lengthe{0.75\linewidth-0.4pt-48pt}
\newlength\lengthf \setlength\lengthf{0.25\linewidth-0.4pt-48pt}

\begin{tblr}{colspec={|Q[\lengthe]|Q[\lengthf]|}, hlines, colsep=24pt}
	A & B \\
\end{tblr}

\end{document}