Check out which ones has some nice features: 142x12 axle, type of BB and headset, front derailleur mount, seatpost size (if you have some seatpost to match with it) etc.
Completely agree. Check for the practicalities which suit you and where you ride. The Pivot has much more mud clearance for example (as far as I am aware) and the replaceable dropouts are nice. Mud clearance and cable routing would be right up there for me due to British mud. Something easy to over look like CF dropouts is something I wouldn't go for on a MTB. I'm sure they last well but for some reason I don't personally like the idea. Are the dropouts full CF on the Scott for example? Little details like that would separate the three options for me.
Plus, the Pivot is much less common than the other two and much more interesting. I wouldn't be too worried about the differences in frame weight as the build doesn't sound very WW and it would probably be better to make the frame as practical and reliable as the rest of the build (i.e. focus on practicalities not just weight).
Personally, I think they are all a hideous rip off compared to a £350 1100g Chinese frame but I realise that isn't for everyone or isn't as bling