It looks like XCode came out with a new update. This issue seems to be persistent where the iTunes Store does not render the update even though it posts it in the "Updates Installed in Last 30 Days" section. After I restart, I am then prompted to update again even though I have already done it twice. Ultimately, the update does not apply when I open XCode (it still shows 7.3, not 7.3.1):
There is only one way! For this you need:
1. Delete the XCode 7.3 from Applications folder
2. Download the Xcode 7.3.1 from here
3. install it manually
That's it))
A possible method of resolving this issue which has worked for people including myself, is as follows:
Close App Store.
Using terminal type the following:
open $TMPDIR/../C
Locate and then delete the folder named com.apple.appstore
Empty Trash folder, if you get an error message saying that files are in use try the following:
sudo rm -rf ~/.Trash/*
Restart Mac.
Once you have restarted your Mac try to install the update.
If this does not work delete Xcode from /Applications and repeat the above method.
来源：https://stackoverflow.com/questions/37030672/xcode-7-3-1-update-not-installing