FLStudio's plugin manager is not up to date with the latest versions of the SDK.
This was taken from the page "VST 3 Locations / Format" in the SDK documentation:
[quote]
On the Windows platform a VST 3 Plug ...
FLStudio 10.
I installed Wavesfactory Trackspacer2 (on Windows7 64bit).
I Installed VST2.4 (32bit) and VST3 (32bit and 64bit) of that plugin.
I opened FLStudio 10, and after I scanned for new plugins(a few times) only the VST2.4 ...