.item:nth-child(1){

  flex-grow: 1;

}

​

.item:nth-child(2){

  flex-grow: 5;

}

​

.item:nth-child(3){

  flex-grow: 2;

}

​

#container {

  display: flex;

  background-color: orange;

}

​

.item {

  background-color: dodgerblue;

  color: white;

  flex-basis: 50px;

}

​

div {

  border: 2px solid black;

  padding: 10px;

  margin-right: 5px;

  box-sizing: border-box;

}