I'm unable to upgrade my OS X to 10.11, due to reasons. XCode 7.3 seems to be the first XCode not to be compatible with Yosemite 10.10.
Has anyone succeeded in getting XCode 7.3 to work on Yosemite? (What's the important update to XCode that has to make it uncompatible with Yosemite?)
Unfortunately no, you can not.
You need OS X 10.11 or later (El Capitan), since March 21st.
So you either update from Yosemite to El Capitan or stick with your current Xcode.
Important? I will say not that much... iOS 9.3 and Swift 2.2, if you are fond of... With the rest of the OSs. The rest is pretty much the same, it's not a major update.