I have created a simple 3-column table, in which the first row has 3 columns, and in the second row the columns are merged into one. The text in the second row is very long.
\documentclass{article}
\usepackage{tabularx}
\begin{document}
\begin{center}
\begin{tabularx}{\linewidth}{|r|X|r|}
\hline
\cellcolor{black!25}Techniques & \cellcolor{black!5}Artifacts & \cellcolor{black!5}Budget \\
\hline
\multicolumn{3}{|c|}{Island Museum analyzes historical artifacts using one or more techniques described below – all but one of which is performed by an outside laboratory – to obtain specific information about an object’s creation. For each type of material listed, the museum uses only the technique described:}
\hline
\end{tabularx}
\end{center}
\end{document}
Obviously, as LaTeX does not automatically wrap text inside a table, the second row text extends past the table edge (and even page edge).
The usually suggested solution to text wrapping inside a table is to set column width (fixed or relative) when beginning the table. I tried that, e.g.
\begin{tabularx}{\linewidth}{|p{0.25cm}|X|p{0.25cm}|}
but that doesn't help.
\multicolumn{3}{|p{\dimexpr\linewidth-2\tabcolsep}|}{... }
c
which is a single line centred cell which is why the text stayed on one line.\usepackage{colortbl}
or even better\usepackage[table]{xcolor}
is missing. Also, a\\
after the\multicolumn
and before the\hline
is missing.