Another option is to download the 11.3 device support at:
iOS 11.3 (15E217)
And don’t forget to remove “(15E217)” from folder name, so it became “11.3”. Restart Xcode afterwards.
Where to paste it according to comment of EdwardM
For those that don’t know where to paste the contents of the zipped file. Find your Applications icon in the dock, right-click, “Open Applications”. The folder will open in Finder. Right-click Xcode > Show Package Contents. Then go to Developer > Platforms > iPhoneOS.platform > DeviceSupport