我删除了flex以放入grid，然后添加了参数grid-template-columns: repeat(3, minmax(0, 1fr));，它创建了3个重复列的行，然后对于内容，我添加了grid-column: span 3 / span 3;，它告诉div占用3个插槽。

body {
        background: #016412;
    }
    .mytabs {
        font-family: 'Times New Roman', Times, serif;
        flex-wrap: wrap;
        max-width: 600px;
        margin: 70px auto;
        padding: 25px;
        display: grid;
        grid-template-columns: repeat(3, minmax(0, 1fr));
    }
    .mytabs input[type="radio"] {
        display: none;
    }
    .mytabs label {
        padding: 25px;
        background: #e2e2e2;
        font-weight: bold;
    }

    .mytabs .tab {
        padding: 20px;
        width: auto;
        background: #fff;
        order: 1;
        display: none;
        grid-column: span 3 / span 3;
    }
    .mytabs .tab h2 {
        font-family: 'Roboto', sans-serif;;
        font-size: 3em;
    }

    .mytabs input[type='radio']:checked + label + .tab {
        display: block;
    }

    .mytabs input[type="radio"]:checked + label {
        background: #fff;
    }