I haven't played online, but heard bad things about it.
The gameplay of BT3 is not the same as BT2, there's no cursor on screen so all your Blasts and Ultimates are done with movement. The story is different, is shorter and "interactive" in some way, which was good for me because BT2 was extremely huge. Yeah, more characters, transformations, the music has improved a lot BTW.
I really liked the game, it was worth for me ($40 I think), but anyway, I recommend you to rent it first.
EDIT: Do you play with GC control or Wiimote-Nunchuck? If it is GC I don't think you'll have any problem, but Wiimote-Nunchuck had some big control changes. For example, in BT2, if you wanted to Hyper Smash the enemy to the left you moved the stick to the left and shake the Wiimote while pressing A. In BT3 you have to move the WiiMote "carefully" from Right to Left while pressing A.
Castlevania Judgment FC: 1161 - 3389 - 1512
3DS Friend Code: 3480-2746-6289
Wii Friend Code: 4268-9719-1932-3069
