Can/Should I point BFD3 to my BFD2/Eco/Zildjian Vault locations?
 
Waiting on the excruciatingly slow download of the BFD3 core library.

I don't have BFD3, but I don't see why not......you'll definitely have more options to choose from. (I think Steve said he had all three installed and used BFD 3 to tweak the other sounds.)   
 
Found this as well: "Any drum sound can be tweaked using the Tech and Model panels"....
http://www.askaudiomag.com/articles/review-fxpansion-bfd3

Not at the machine right now but IIRC:
 
After you have BFD3 installed you can click on the 'Tools' dropdown list, select 'Setup Content Locations' and then you can setup a path to any content that BFD3 did not pickup during the install. Do a rescan and you should have it!

I did and it sees them. I don't know if this is true for everyone but BFD 3 will categorize them a little differently then in BFD2 for expansion packs.  

BFD3 likes to ingest the old BFD libraries just fine. I bought an expansion (that turned out to suck) and it ingested and worked with it straight away. There is a tiny loading delay when you load one for the first time as it does all the import but other than that works as normal.

Thanks guys.

Sycraft
There is a tiny loading delay when you load one for the first time as it does all the import but other than that works as normal.

as in, copies old file a the new BFD3 location?
 
IOW will I eventually have BFD3 copies of all BFD2 kits?

Doesn't copy the sound data, I think it just makes a new kit data file. I'm not really sure, I just noticed there is an import process when you first open something. Doesn't do it until you actually try and load something to be used near as I can tell.

Yes you most certainly should, as others have said.

BFD 3 will love you for it 😜