It seems to me, wehn building AAX and VST3 in one go in windows (and maybe this is whats wrong to begin with?) the post build copy for the vst3 version fails, as the source file for the copy command is not found.
The source file in the copy command is named xy.dll, but it should be "xy.aaxplugin" for an vst3 and AAX build.
I took a look at the introjucer code and could fix this.
Or is this intended?
My suggestion would be to always build to "xy.dll" and copy from there instead of using "xy.aaxplugin" (which seems to be the case for aax only?).
edit: oh btw. i'm not on the latest JUCE from the repo, judging from the change logs this might still be an issue.