You are not exporting at 3x, it says you are "Designing at" 3x, so the 1x and 2x will come out smaller.
I know it's confusing, but "Designed at" should be set to "1x" in your case, so Xd will export the bigger 2x and 3x from that. When you set it to 3x, it works backwards from that, and divides it to make the smaller 1x and 2x resolutions.
You might wonder why it's made like that, why would you design at 3x? An example would be a mobile game like this.
In that situation, you would be using a lot of raster graphics made in Photoshop, and you want to "Design at" the scaled 3x resolution, to ensure everything is crisp on the retina devices. Xd would then scale down the assets for the smaller resolutions on export. In your case, with vector graphics, you design at 1x, since scaling them up isn't a problem.
Otherwise, quality looks good, so I assume you were talking about these smaller resolutions. Let me know if I misundrestood something 🙂
Cheers!