OK, we obviously checked that VST3 compiles, so that could be a bug.
What plug-in are you compiling exactly? Can you reproduce the problem with the JUCE audio plug-in demo without further modifications? Can you try to re-download the VST3 SDK, maybe something is broken there?
Also: What Xcode version? What OS X SDK version? What OS X Deployment target? What VST3 SDK version? (I tested it with Version 3.6.5 and it works just fine). Do you compile with C++11 or C++98?