.container{
display:glex;
width:100%; //или нужная
}
.container div{
flex-grow: 1;
}
<div class="parent">
<div class="child"> </div>
<div class="child"> </div>
<div class="child"> </div>
</div>
.parent {
display: flex;
justify-content: stretch;
.child {
flex:1;
margin:2px 1px;
border:1px solid #888;
background: #ececec;
}
}