border: 1px solid #c3c3c3;
#main div:nth-of-type(1) {flex-grow: 1;}
#main div:nth-of-type(2) {flex-grow: 1;}
#main div:nth-of-type(3) {flex-grow: 1;}
<div style="background-color:coral;"></div>
<div style="background-color:lightblue;" id="myBlueDiv"></div>
<div style="background-color:khaki;"></div>
<p>点击“运行”按钮设置蓝色 div 元素的 flexGrow 属性的值为 "5"。</p>
<button onclick="myFunction()">运行</button>
document.getElementById("myBlueDiv").style.flexGrow = "5";
<p><b>注意:</b>Internet Explorer 和 Safari 不支持 flexGrow 属性。</p>