Currently, the Windows Slapper installer selects what should be the default 64-bit VST2 folder to install the VST3 plugin, and then it also defaults to the same folder for VST2. The common way VST3 plugin installers are handled with every other VST3 plugin I am familiar with is to not even give the user an option of folder selection for the VST3 version of a plugin. The only correct location for a 64-bit VST3 plugin to exist is:
C:\Program Files\Common Files\VST3
As Steinberg’s site states, all 64-bit VST3 plugins must live in this folder in order to work.