Edit in JSFiddle

div > div:nth-child(1) {
    flex-grow: 1;  
}
div > div:nth-child(2) {
    flex-grow: 3;  
}
div > div:nth-child(3) {
    flex-grow: 1;  
}
div > div:nth-child(4) {
    flex-grow: 2;  
}
div > div:nth-child(5) {
    flex-grow: 1;  
}

.flex {
  display:flex;
  flex-direction:row;
  width:500px;
}
div {
  border:1px solid #cccccc;
  margin:5px;
  padding:5px;
}
div > div {
  background:#ffcc99;
}

將 Item2變成其它 flex item 的三倍，Item 4 變兩倍
<div id="container" class="flex">
   <div>Item1</div>
   <div>Item2</div>   
   <div>Item3</div>  
   <div>Item4</div>  
   <div>Item5</div>    
</div>