- As far as I know, the third sentence is the best choice. Hope other native speakers will agree with me.
Thanks very much! You have explained it very well!It depends on context. The first two are more likely as a follow up in an ongoing conversation and might, for example, express having thought or not having thought about the matter before:
A. "Don't forget you'll need plenty to read while you're on holiday."
B1: "[I never thought of that!] I'll buy some books tomorrow."
B2: "[I've thought about that already.] I'm buying some books tomorrow."
The third might be more likely to be said in opening a new topic.
Of course, these are just some possibilities.