Shalom rabbi, During these 3 weeks can I study mishnah or torah, even knowing that when I study mishnah it gives me a lot of joy?
It is beautiful that you get so much joy while learning, that is the way it is supposed to be!
Regarding your question, there is nothing wrong with having joy from torah study during the three weeks. We oly abstain frm learning joyous things on ever Tisha B’av from after chatzos and on Tisha B’av itself. Even then, we are allowed to study things that are sad to learn such as the gemoras that talk about the destrcution of the holy temple, halachos of mourning etc.
O:CH 553-2, 554-1,2.