CSS:divは平均していくつのブロックに分けます

css:
法一：浮动布局+百分比

        .container{
            overflow: hidden;
            zoom:1;
            width:500px;
            height:200px;
            border:1px solid red;
        }
        .childDiv{
            float:left;
            width:33.3%;
            height:100%;
            border:1px solid red;
            background:greenyellow;
        }

        .container{
            font-size: 0;
            overflow: hidden;
            width:500px;
            height:200px;
            border:1px solid lightblue;
        }
        .childDiv{
            display: inline-block;
            width: 33.3%;
            height:100%;
            border:1px solid gray;
        }

        .container{
            width:500px;
            height:200px;
            display: table;
            border:1px solid lightblue;
        }
        .childDiv{
            display: table-cell;
            border:1px solid gray;
        }

        .container{
            width:500px;
            height:200px;
            display: flex;
            border:1px solid lightblue;
        }
        .childDiv{
            flex:1;
            border:1px solid gray;
        }