When is Shimano Di3 going to be released? And 12-speed Di2 for MTB?

Di3....?

Sorry, it doesn't work that way. Di2 stands for Digital Integrated Intelligence. Of course, Shimano could add another I to that and call it Digital Integrated Intelligence Improved (Di3)… but it’s more likely that they will just increase the existing model / product codes.

12-speed XT / XTR Di2

When Shimano plans to announce a 12-speed MTB Di2 update, no one really knows for sure. However, my best bet would be 2025.