.container{
        display: flex;
        width: 500px;
        height: 200px;
    }
    .left{
        flex:1;
        background: red;
    }
    .middle{
        flex:1;
        background: green;
    }
    .right{
        flex:1;
        background: blue;
    }
<div class="container">
    <div class="left"></div>
    <div class="middle"></div>
    <div class="right"></div>
</div>

    .container{
        display: flex;
        height: 300px;
    }
    .left{
        flex: 0 0 100px;
        background-color: red;
    }
    .middle{
        flex: 0 0 100px;
        background-color: green;
    }
    .right{
        flex:1;
        background-color: blue;
    }
  <div class="container">
    <div class="left">qqq</div>
    <div class="middle">qqq</div>
    <div class="right">wwww</div>
</div>

  .container{
        display: flex;
        height: 300px;
    }
    .left{
        width: 100px;
        background-color: red;
    }
    .middle{
        flex: 1;
        background-color: green;
    }
    .right{
       width: 100px;
        background-color: blue;
    }
   <div class="container">
    <div class="left">qqq</div>
    <div class="middle">qqq</div>
    <div class="right">wwww</div>
</div>

.container{
        display: flex;
        height: 300px;
        width: 300px;
        flex-direction: column;
    }
    .row{
        display: flex;
        height: 100px;
    }
    .left{
        flex: 1;
        height: 100px;
        border: 1px solid red;
    }
    .middle{
        flex: 1;
        height: 100px;
        border: 1px solid green;
    }
    .right{
        flex: 1;
        height: 100px;
        border: 1px solid blue;
    }
    <div class="container">
    <div class="row">
        <div class="left"></div>
        <div class="middle"></div>
        <div class="right"></div>
    </div>
    <div class="row">
        <div class="left"></div>
        <div class="middle"></div>
        <div class="right"></div>
    </div>
    <div class="row">
        <div class="left"></div>
        <div class="middle"></div>
        <div class="right"></div>
    </div>
</div>

  *{
        margin:0;
        padding:0;
    }
    .container{
        display: flex;
        flex-direction: column;
        min-height: 100vh;
        justify-content: space-between;
    }
    .header{
        background: red;
        flex: 0 0 100px;
    }
    .content{
        display: flex;
        flex:1;
    }
    .content-left{
        flex: 0 0 100px;
        background: green;
    }
    .content-right{
        flex: 0 0 100px;
        background: pink;
    }
    .content-middle{
        flex:1;
    }
    .footer{
        background: yellow;
        flex: 0 0 100px;
    }
    <div class="container">
    <div class="header">Header</div>
    <div class="content">
        <div class="content-left">Left</div>
        <div class="content-middle">Center</div>
        <div class="content-right">Right</div>
    </div>
    <div class="footer">Footer</div>
</div>