JSFiddle - React, Tailwind, and code Playground

HTML

<table  class="tri">
    <thead>
    <tr>
        <th>one</th>
        <th>two</th>
        <th colspan="2">three</th>
        <th>one</th>
        <th>two</th>
        <th>three</th>
    </tr>
    </thead>
    <tbody>
    <tr>
        <td>one</td>
        <td>two</td>
        <td>three</td>
        <td>one</td>
        <td>two</td>
        <td>three</td>
    </tr>
    <tr>
        <td>one</td>
        <td>two</td>
        <td>three</td>
        <td>one</td>
        <td>two</td>
        <td>three</td>
    </tr>
    <tr>
        <td>one</td>
        <td>two</td>
        <td>three</td>
        <td>one</td>
        <td>two</td>
        <td>three</td>
    </tr>
    </tbody>
</table>

CSS

*{border: solid 1px red; }
.tri  th:nth-child(2){background: #000000;}