html,body{height:100%} .wraper{
display:flex;
flex-direction:column;
height:100%;
flex-wrap: nowrap; } div{
border:1px solid red;
} .top{
height:20px;
} .mid{
border:1px solid green;
flex:1;
} .bottom{
height:20px;
}
<div class="wraper">
<div class="top"></div>
<div class="mid"></div>
<div class="bottom"></div>
</div>