Just use another container to wrap last two divs. Don't forget to use CSS prefixes.
#productShowcaseContainer {_x000D_
display: flex;_x000D_
flex-direction: column;_x000D_
height: 600px;_x000D_
width: 580px;_x000D_
background-color: rgb(240, 240, 240);_x000D_
}_x000D_
_x000D_
#productShowcaseTitle {_x000D_
height: 100px;_x000D_
background-color: rgb(200, 200, 200);_x000D_
}_x000D_
_x000D_
#anotherContainer{_x000D_
display: flex;_x000D_
height: 100%;_x000D_
}_x000D_
_x000D_
#productShowcaseDetail {_x000D_
background-color: red;_x000D_
flex: 4;_x000D_
}_x000D_
_x000D_
#productShowcaseThumbnailContainer {_x000D_
background-color: blue;_x000D_
flex: 1;_x000D_
}
_x000D_
<div id="productShowcaseContainer">_x000D_
<div id="productShowcaseTitle">1</div>_x000D_
<div id="anotherContainer">_x000D_
<div id="productShowcaseDetail">2</div>_x000D_
<div id="productShowcaseThumbnailContainer">3</div>_x000D_
</div>_x000D_
</div>
_x000D_