<div class="flex__container">
  <div class="flex__item">
    <select id="flex1">
      <option value="none">none</option>
      <option value="auto">auto</option>
      <option value="initial">initial</option>
      <option value="2">2</option>
      <option value="30vw">30vw</option>
    </select>
  </div>
  <div class="flex__item">
    <select id="flex2">
      <option value="none">none</option>
      <option value="auto">auto</option>
      <option value="initial">initial</option>
      <option value="2">2</option>
      <option value="30vw">30vw</option>
    </select>
  </div>
  <div class="flex__item">
    <select id="flex3">
      <option value="none">none</option>
      <option value="auto">auto</option>
      <option value="initial">initial</option>
      <option value="2">2</option>
      <option value="30vw">30vw</option>
    </select>
  </div>
</div>
@import url("https://fonts.googleapis.com/css?family=Gochi+Hand");

* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

body {
  width: 100vw;
  min-height: 100vh;
  margin: 0;
  background-color: #291642;
  font-family: "Gochi Hand", sans-serif;
  font-size: 100%;
  letter-spacing: 0.1rem;
  color: #fff;
  display: flex;
  justify-content: center;
  align-items: center;
}

:root {
  --flex1: none;
  --flex2: none;
  --flex3: none;
}

.flex__container {
  inline-size: 80vw;
  block-size: 40vh;
  
  min-inline-size: calc(300px + 20vh);
  max-inline-size: 96vw;
  padding: 2vh;
  border: 1px dashed #f36;
  overflow: hidden;
  resize: horizontal;
  
  display: flex;
  align-items: stretch;
}

.flex__item {
  padding: 2vh;
  border-radius: 4px;
  display: flex;
  justify-content: center;
  align-items: center;
}


.flex__item:nth-child(1) {
  background-color: #f36;
  flex: var(--flex1);
}

.flex__item:nth-child(2) {
  background-color: #cddc39;
  flex: var(--flex2);
  margin: 0 2vh;
}

.flex__item:nth-child(3) {
  background-color: #2196f3;
  flex: var(--flex3);
}

select {
  font-size: .9em;
  max-width: 100px;
  min-width: 100px;
}
View Compiled
const selects = document.querySelectorAll("select");

selects.forEach((select) =>
  select.addEventListener("change", (e) => {
    document.documentElement.style.setProperty(
      `--${e.target.id}`,
      e.target.value
    );
  })
);

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.