*{margin:0;padding:0;}
.box{border:1px solid #000;display:flex;}
.box div{border:1px solid red;flex:1}
.box div:nth-child(2){flex:auto;width:800px;text-align:center;}

.box div:nth-child(1) p{
  padding-left:20px;
}
.box div:nth-child(3) p{
  padding-right:10px;
}