can't download Xcode 9.3, disk is full in fact it is not
If there is a partial download begun of the app/purchase, deleting the partially downloaded file and starting from scratch may help. (Check in your Applications folder or downloads area for this partial file. It may end in .partial or .part.)

I also had to download the installer from the apple site, not through the App Store, when I recently installed xcode:
https://developer.apple.com/xcode/

For reference, older downloads may be available here: 
https://developer.apple.com/download/more
Warning: if you aren't logged in when you visit the 'more' link, it will redirect you to the latest download site once login is complete. Don't be fooled and just re-enter this URL again to go to the archive.