JSFiddle - React, Tailwind, and code Playground

CSS

html {
    height: 100%;
}
body {
  margin:0; 
  padding:0;
  height:100%;    
}

.my-flex-container{
    display: flex;
    width : 100%;
    height: 100%;
    border: 1px solid #000;
    flex-direction: column;
    box-sizing: border-box;
    overflow: hidden;
}

.b1 {
    background-color: #ccc;
    flex-shrink: 0;
}

.b2 {
    background-color: #bbb;
    flex-shrink: 0;
}

.b3 {
    background-color: #aaa;
    flex-grow: 1;
    overflow-y: auto;
}