Pro Tools does not support VST3 out of the box. You'll have to use an AAX to VST3 wrapper (for example, BlueCat Audio's Patchwork) to make it work. (I have no experience with this however, so YMMV)
In the past I've tried getting access to the AAX SDK, but their terms are incredibly unfriendly to independent developers, so I haven't been able to.
