Just use percentage widths and fixed table layout:
<table>
<tr>
<td>1</td>
<td>2</td>
<td>3</td>
</tr>
</table>
with
table { table-layout: fixed; }
td { width: 33%; }
Fixed table layout is important as otherwise the browser will adjust the widths as it sees fit if the contents don't fit ie the widths are otherwise a suggestion not a rule without fixed table layout.
Obviously, adjust the CSS to fit your circumstances, which usually means applying the styling only to a tables with a given class or possibly with a given ID.