Here's a comparison shot of all three.
Brooks comparison by
jstewse, on Flickr
I have the b17 Standard on my bike with a Nitto Technomic, and I think that's the best combination. It's a much better saddle for a more upright position, I wouldn't put it on a bike with bar to saddle drop. For that type of bicycle the b17 is the most comfortable.
As for the other two, I've ridden the Pro on the bike I mentioned above, and it was okay but the wider rear platform of the b17 is just so much better. On a more hunched over sort of bike, I think I like the b17 Narrow better. It's a bit softer than the Pro, and despite it being considerably narrower, I find it more enjoyable. Of course though, as we like to say YMMV.
@Rootboy, did you happen contact Brooks about it? Care to share the details?