theadnot h3, if you use the element h3inside td, this is absolutely normal, but if you use h3, since the direct child in tris invalid.
To use thead, you can use it as follows
<table>
<thead>
<tr>
<th>This is table head and not h3</th>
</tr>
</thead>
<tfoot>
<tr>
<td>Foot Cell</td>
</tr>
</tfoot>
<tbody>
<tr>
<td>
<h3>It completely valid to put your h3 here</h3>
Table cell
<p>You can also use p tag here
</td>
</tr>
</tbody>
</table>
But if you do something like this, it’s not valid
<table>
<tr>
<h3>It invalid</h3>
<td>This is a cell</td>
</tr>
</table>
Strike>
, .