body {
margin: 0;
}
.main {
width: 100vw;
height: 100vh;
background: skyblue;
display: flex;
align-items: center;
}
.main div:nth-of-type(1) {
width: 100px;
height: 100px;
background: pink;
}
.main div:nth-of-type(2) {
flex-grow: 1;
background: greenyellow;
align-self: stretch;
}
.main div:nth-of-type(3) {
width: 100px;
height: 100px;
background: pink;
}