So I'm not sure if this has already been discussed or not but I have a possible theory about the third Bondsmith spren. I'm not sure if this theory even makes sense so let me know if you think I'm off base here.
In the chapter 67 epigraph it is stated that this generation of radiants only had one Bondsmith.
"This generation has had only one Bondsmith, and some blame the divisions among us upon this fact. The true problem is far deeper. I believe that Honor himself is changing."
24-18 smokestone
I'm assuming that this epigraph takes place around the time of the recreance (and if I'm wrong then this theory doesn't really makes sense) and that this bondsmith was not bonded to the stormfather or the nightwatcher. If this is the case do you think it's possible that this bondsmith was one of the radiants who broke their oath during the recreance and in turn killed the third bondsmith spren? If that is the case then we might never actually see more than two bondsmiths because only two of the three spren are still alive.
Again I'm not sure if this even makes sense, so let me know what you think!