I wouldn’t bother with 7.3. As long as you’re downloading the public release version of xcode 7.3.1 it will contain the iOS 9.3 full version (not beta).
I’d also remove the beta version of xcode to save headaches whilst you’re waiting for the download to finish
I already checked back the history. I downloaded XCode 7.3.1 from https://developer.apple.com/downloads/ and it only contains iOS 9.3 (beta) version. I downloaded it on yesterday.
I installed XCode 7.3 from link and it still shows 9.3 (beta) version. And cannot choose 9.2, it shows as (install or xcode-select XCode 7.2 to enable). Damn, I already downloaded 2 times 
Is there a reason you can’t update to a modern daily build?
I tried to update it, and I got 7.3.1 before but it only shows 9.3 Beta.
I still don’t get that “9.2 install or xcode-select XCode 7.2 to enable it” and I got another option as 9.3 beta to build the app. Sorry for the noob question because it is my first time touching Mac OSX and I was with Linux since 2009. So I have no idea about Mac OSX. I am using Ubuntu with MATE DE, Mac OSX El Capitan Guest in VMWare, and running Corona, XCode inside it. My current XCode is 7.3 and I installed 7.3.1 before. And both shows iOS 9.3 beta only. How I should do it?
Btw, my Corona version is 2016.2830.
@Rob, I didn’t know that Corona has daily build 
Yes, I think I know what is the error. XCode 7.3 has iOS 9.3 (full version) not the beta one. But Corona build 2016.2830. cannot detect as it was built for iOS 9.2, so it detects 9.3 as the beta version.
The way software updates happen is they happen in new builds. We can’t go back and patch 2830. So once it’s released, it is what it is. iOS 9.3 came out after the release. We try to add in new iOS SDK’s when its reasonable. Apple is the one making you match the SDK build with Xcode. In a couple of weeks iOS 10 Beta 1 will drop with Xcode 8. People need to stay with Xcode 7.3 if they want to build releasable versions.
Rob