New Twiddle

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  init() {
    this._super(...arguments);
    this.set('card', Ember.Object.create({ number: 12 }));
  },
  
  actions: {
    updateNumber(event) {
      let userInput = event.target.value;
      let card = this.get('card');
      if (userInput.length <= 3) {
        card.set('number', userInput);
      }
      card.notifyPropertyChange('number');
    },
  },
});

templates.application.hbs

<input
      value={{card.number}}
      oninput={{action "updateNumber"}}>

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.4.2"
  }
}