I'm not too good at these gAmes but I'm good at regular Budokai 3. Will this be too difficult for me? I just like to do the story mode, unlock every character, then duel the computer. Is the story mode hard and is the CPU frustrating?
"Is this game too hard?"... this depends entirely on how far you are willing to push yourself. You can play it normally, and I'm sure you'll do fine. I remember playing B3... it was also a struggle but we made it through, right? However, if you choose to reach the level of gameplay as a lot of these other Tenkaichi 3 pros, you should expect a difficult path ahead.
Still, as far as the topic at hand. You seem to be asking for the general ideas, so I'll say that you'll do fine. The game is pretty user friendly, but if you have no Tenkaichi experience (T1, T2), then you should know it is definitely a new gaming concept to wrap your head around. Adaptation is key, as with any new game. Good luck!
Also, keep this in mind. Tenkaichi 3 has been out for a very long time now. If you have questions, check on YouTube for other Tenkaichi 3 pros. Most of them (who speak your language, lol) are more than willing to help with any questions regarding progressive game-play ability. The more time you spend with the game and online videos, the faster you'll learn who these people are.