To be fair, of those three only Tobias really counts as being a sue type trope - ridiculously overpowered and able to take down the league as if it were nothing. Cameron certainly benefits from plot connivance as it's conceivable that but for Riolu evolving he probably wouldn't have won. But that's conceivable within cannon given Riolu evolved from friendship, and the series up to that point has seen several major battles turn on evolution. That is itself ultimately less plot convince than a very strong character entering the league on a fly simply to battle another trainer (who would have agreed to battle him anyhow most likely). Zoe is relatively consistent and believable - she was just simply a strong and experienced co-ordinate just like the Milotic trainer had been in AG.
There are many divisions of being a Sue/Stu. Here are some shamelessly copied and pasted from tv tropes
Anti-Sue — I'm genuinely useless, but everybody still loves me!
Black Hole Sue — Everything is about me!
Copy Cat Sue — I'm just like my favorite character, but even kewler!
Fixer Sue — No, that's not how it's supposed to go!
God-Mode Sue — Power overwhelming!
Jerk Sue — I'm a complete and utter ***** and I have constant PMS...love me!
Mary Tzu —I knew you would do that. In fact, I knew you would do that before I even met you, cuz I'm JUST THAT GOOD!
Parody Sue — Why don't they fall for my buxom charms?
Possession Sue — My favourite character is an even better version of me!
Purity Sue — Love me!
Relationship Sue — You're my boyfriend now!
Sympathetic Sue — Feel sorry for me!
Thirty Sue Pileup — We are Legion.
Villain Sue — I have you now, my beautiful slaves! Ahahahahahahaha!