Here’s an example:
Always prepare the ‘images’ assets which are not vector separately, meaning create three different versions of it manually instead of using just the export panel.
The way you have done above in the export panel, Sketch will increase the size of your asset for @2x and @3x, which will lead to a bad quality image.
In my example above, as you can see I have selected 150x150 image and in export panel you see me adding the suffix @3x manually. Then I reduced the size of the image to 100x100 and added the suffix @2x to it and reduced it again to 50x50 and added no suffix. So when I export I have to export these three images instead of just selecting one and letting Sketch create three versions out of it.