有8个图片资源的url，已经存储在数组urls中。

urls类似于['https://image1.png', 'https://image2.png', ....]

而且已经有一个函数function loadImg，输入一个url链接，返回一个Promise，该Promise在图片下载完成的时候resolve，下载失败则reject。

但有一个要求，任何时刻同时下载的链接数量不可以超过3个。

请写一段代码实现这个需求，要求尽可能快速地将所有图片下载完成。

复制
1var urls = [
2  "https://hexo-blog-1256114407.cos.ap-shenzhen-fsi.myqcloud.com/AboutMe-painting1.png",
3  "https://hexo-blog-1256114407.cos.ap-shenzhen-fsi.myqcloud.com/AboutMe-painting2.png",
4  "https://hexo-blog-1256114407.cos.ap-shenzhen-fsi.myqcloud.com/AboutMe-painting3.png",
5  "https://hexo-blog-1256114407.cos.ap-shenzhen-fsi.myqcloud.com/AboutMe-painting4.png",
6  "https://hexo-blog-1256114407.cos.ap-shenzhen-fsi.myqcloud.com/AboutMe-painting5.png",
7  "https://hexo-blog-1256114407.cos.ap-shenzhen-fsi.myqcloud.com/bpmn6.png",
8  "https://hexo-blog-1256114407.cos.ap-shenzhen-fsi.myqcloud.com/bpmn7.png",
9  "https://hexo-blog-1256114407.cos.ap-shenzhen-fsi.myqcloud.com/bpmn8.png",
10];
11function loadImg(url) {
12  return new Promise((resolve, reject) => {
13    const img = new Image();
14    img.onload = function() {
15      console.log("一张图片加载完成");
16      resolve(img);
17    };
18    img.onerror = function() {
19    	reject(new Error('Could not load image at' + url));
20    };
21    img.src = url;
22  });