I generally find riku more annoying then the triplets as well.
If you keep moving around, waka can't generally land hits on you. And selphie broadcasts her spin attack with an audible 'get ready', while her regular attacks are, again, pretty avoidable if you just keep moving. Lock onto tidus first and just wail on him, being sure to keep moving around, and be ready to switch target to Selphie and block if you hear her call out. After tidus drops, selphie isn't too hard, again stick and move to avoid hits from wakka. Once both of the melee fighters are down, then wakka's just wakka. After training against each of them individually, you should be able to finish wakka off even if you only have a sliver of health left at that point.
It is important to train against each of them individually, though, so that you know what their cues are, and so that you can beat wakka without losing any health, because you likely won't have much left by the time you've finished the other two off.
Riku, on the other hand, is a lot faster, shifts his attacks up more, is more prone to countering, and in general doesn't broadcast his attacks quite so readily. Not that the trio is easy, mind, but I just always have more trouble with Riku.
