圣杯布局

• 左右固定宽度,中间自适应
• div:nth-child(1)

section {display: flex;width: 60%;height: 150px;background-color: pink;margin: 0 auto;
}section div:nth-child(1) {width: 100px;height: 150px;background-color: red;
}section div:nth-child(2) {flex: 1;background-color: green;
}section div:nth-child(3) {width: 100px;height: 150px;background-color: blue;
}