dwgame01/assets/scripts/UI/Background.js

cc.Class({
    extends: cc.Component,

    properties: {
        bgFrames: [cc.SpriteFrame],
        bgWrap: cc.Node
    },
    
    onLoad () {
        let ran = Math.floor(cc.random0To1() * 3)
        this.bgWrap.getComponent(cc.Sprite).spriteFrame = this.bgFrames[ran];
    },
    
});