<header>header</header>
<section>
  <aside>aside long content...</aside>
  <main>main long content...</main>
  <aside>aside long content...</aside>
</section>
<footer>footer</footer>

body {
  margin: 0;
  height: 100vh;
  display: flex;
  flex-direction: column;
}

header, footer {
  height: 50px;
  min-height: 50px;
  background: black;
  color: #fff;
}

section {
  flex-grow: 1;
  display: flex;
}

section aside  {
  width: 100px;
  background: #ccc;
  height:100%;
  overflow:scroll;
}

section main {
  overflow:scroll;
  flex-grow: 1;
}

body {
  margin: 0;
  height: 100vh;
  display: flex;
  flex-direction: column;
}

header, footer {
  height: 50px;
  min-height: 50px;
  background: black;
  color: #fff;
  transition-property: height;
  transition-duration: 0.2s;
  transition-timing-function: linear;
}

section {
  flex-grow: 1;
  display: flex;
  height:0px; /* <== for scrolls in aside */
}

aside  {
  width: 100px;
  background: #ccc;
  overflow:auto;
  min-height: 0;
  transition-property: width;
  transition-duration: 0.2s;
  transition-timing-function: linear;
}

section main {
  overflow:auto;
  flex-grow: 1;

}
footer:hover
{
    height:200px;
}
aside:hover
{
    width:200px;
}

<header>header</header>
<section>
  <aside><p>main long content...</p><p>main long content...</p><p>main long content...</p><p>main long content...</p><p>main long content...</p><p>main long content...</p><p>main long content...</p><p>main long content...</p><p>main long content...</p><p>main long content...</p></aside>
  <main>main long content...</main>
  <aside><p>main long content...</p><p>main long content...</p><p>main long content...</p><p>main long content...</p><p>main long content...</p><p>main long content...</p><p>main long content...</p><p>main long content...</p><p>main long content...</p><p>main long content...</p><p>main long content...</p><p>main long content...</p></aside>
</section>
<footer>footer</footer>