table
{
table-layout: fixed;
width: 100%;
white-space: nowrap;
}
.th-1column {
width:10%;
}
.th-2column {
width:20%;
}
.th-3column {
width:70%;
}
<table>
<thead>
<tr>
<th class="th-1column">1 столбец</th>
<th class="th-2column">2 столбец</th>
<th class="th-3column">3 столбец</th>
</tr>
<tbody>
<tr>
<td>Вася</td>
<td>Пупкин</td>
<td>go@to.ass</td>
</tr>
</tbody>
</thead>
</table>