Eventually, probably all 3. It’s harder to guess which one will turn black first, maybe all 3 at once since it might be enough to master this ability with a single sword to use it with any other weapon. Otherwise, we can make a case for any of them :
- doing it with Wado first would be symbolically powerful because this sword represents his dream and promise to Kuina ( however the same arguments can be used to say it will be the last one to turn black, perhaps in his fight against Mihawk)
- doing it with Sandai first would make sense because it’s his only third grade sword so it’s the one that needs an upgrade the most
- Enma is Oden’s weapon and Zoro will face Kaido in this arc with this sword, the sword that once injured Kaido, so it would be symbolically powerful to turn it black and inflict a serious injury to Kaido with it
Enma has the strongest case, I believe.