今日は！ 'I will" has the highest possibility, "i'm going" the second highest possibility ('I will' and 'I'm going' is kind of the same, i'll explain in a bit) and "I plan" has the lowest possibility. Out of the three you listed, "I will" has the highest possibility, as "will" is definite. It means that your action, absolutely will happen nothing will change that. no matter what happens, tennis is going to happen. "I'm going" is like "i will", they both have the similar possibility but 'i'm going" is a nicer way of saying 'I will'. "I will" you could say if you are determined to do something, "I'm going" you would use if you are determined to do something but you could still change your mind (Because 'going" is an action word, it's like you are saying 'i'm on my way to tennis" but this could mean that on your way to tennis you could change your mind and go somewhere else) Lastly, "I plan' has the lowest possibility. Saying "I plan to play tennis" means that you would like to play tennis but you're not sure if you will have time. 'Planning' is not definite because you don't know if that action will be done; 'will' and 'going' both mean you are on your way to doing that action so you are most likely to do that action, where as planning is just an idea you have, it could happen but it's likely that it won't happen. I hope this helped ^-^ if you need anymore help, i'm happy to help you. ご幸運を祈ります! :P

I will play tennis. - decision made at the moment of speaking I'm going to / I plan to play tennis. - talking about a plan, decision made earlier "I plan to" is more formal.