<form >
  <p>Three letters country code!</p>
  <label for="input">Country Code:</label>
  <input type="text" id="input" pattern="[A-Za-z]{3}" title="Three letters country code." required>
  <input type="submit">
</form>

body{
  height: 90vh;
  display: flex;
  align-items: center;
  justify-content: center;
}