Xcode not supported for iOS 11.3 by Xcode 9.2 needed 9.3

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

I have the same problem, OS Sierra trying to run iPhone SE with 10.3 using Xcode 9.2 and I don't have the resources to update to High Sierra and Xcode 9.3.

Solution:

1. Find your friends who have Xcode 9.3
2. Ask them to right click on Xcode, Show Package Contents then navigate to Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport/11.3 (15E217)
3. Copy the 11.3 (15E217) folder to your old Xcode (9.2) with the same file hierarchy

Now you have Xcode 9.2 with iOS 11.3.

I just give you a device support file for iOS 11.4 (15F79).which just on XCode version 9.4 (9F1027a)

Updated on 5 Oct 2018:

• Xcode 10.0 (10A255) - iOS 12.0 (16A366) [Updated 21 Sep: @NikBurns confirmed that works]
• Xcode 10.1 beta 2 (10O35n) - iOS 12.1 (16B5068g) [Please give me feedback if that works]