The pocket companion states that there are three spren that can bond a person to make them into a bondsmith, the Stormfather being one of them. As far as I recall the books implied that the number was low, and implied heavily that it was around that number in an epigraph, but didn't actually have a straight confirmation. So, should I take that as canon?
Yes, you can take that as canon. They came to me for that information.
Sweet. I guess it hasn't been canonized which three spren these are? I seem to recall that the prevailing theory on 17th Shard was that Nightwatcher was one of them, and the third was that weird spren with too many faces that Axies the Collector looked at in the tWoK interlude.