Download older version of Xcode?
Try Google Chrome and curl:


Open Terminal
Open Google Chrome and go to Developer Apple downloads.
Scroll down to Xcode 7.3.1 and click the + sign.

 
open Google Chrome's Developer Tools (altcmdI) > Network panel
Click on Xcode 7.3.1.dmg to start the Xcode download
Right-click Xcode 7.3.1.dmg in the network panel > Copy as cURL


Stop the Xcode 7.3.1.dmg download on Google Chrome's Downloads page or in the status bar
Paste the content of the pasteboard to the Terminal window with cmdV and add -o Xcode7.3.1.dmg. Then hit the enter key.

Example:

curl 'http://adcdownload.apple.com/Developer_Tools/Xcode_7.3.1/Xcode_7.3.1.dmg' -H 'Accept-Encoding: gzip, deflate, ... &lt;redacted&gt; ...; as_dc=nwk; ADCDownloadAuth=&lt;redacted&gt;' -H 'Connection: keep-alive' --compressed -o Xcode7.3.1.dmg


If you add additionally -L -C - at the end (after the suggested -o Xcode7.3.1.dmg), you can resume previous downloads (i.e. download interrupted server-side).


I hope this works for you.