JSFiddle - React, Tailwind, and code Playground

HTML

<table class="dummyclass">
    <tbody>
        <tr class="This_should_get_a_background_color">
            <th>Dummytext</th>
            <td><a href="#">Dummy dummy</a>
            </td>
        </tr>
        <tr class="This_should_get_a_background_color">
            <th>Dumminho</th>
            <td>Golazo</td>
        </tr>
        <tr class="This_should_get_a_background_color">
            <th>Great game</th>
            <td>yesterday</td>
        </tr>
        <tr class="no-background-please">
            <th>Dummytext</th>
            <td><a href="#">Dummy dummy</a>
            </td>
        </tr>
        <tr class="no-background-please">
            <th>Dumminho</th>
            <td>Golazo</td>
        </tr>
        <tr class="no-background-please">
            <th>Great game</th>
            <td>yesterday</td>
        </tr>
        <tr class="This_should_get_a_background_color">
            <th>Dummytext</th>
            <td><a href="#">Dummy dummy</a>
            </td>
        </tr>
        <tr class="This_should_get_a_background_color">
            <th>Dumminho</th>
            <td>Golazo</td>
        </tr>
        <tr class="This_should_get_a_background_color">
            <th>Great game</th>
            <td>yesterday</td>
        </tr>
    </tbody>
</table>

CSS

tr:nth-child(6n + 3), 
tr:nth-child(6n + 2), 
tr:nth-child(6n + 1) {
    background: #f00;
}