So if I re-roll three different champions, I get Camille?

You can only reroll into a champion you don't own, so if she's the only one left, yes it will be 100% guaranteed you get Camille. Once you own them all it creates permanents that just sit in your inventory, which can be disenchanted or used for other crafting (they aren't worth as much as 3 shards).

If she is all that you still do not own, that is correct. It's how I plan to get all the 6300 champions (I have to buy Vladimir yet) for a total savings of like 18000 IP because I don't own about 15 6300 champs yet. That's sooo many games saved! Though I won't have any control over which champion I get, but I'll make that sacrifice.

Assuming you're talking about Champion Shards and Hextech Carfting: Yes. Once you own all Champions you can get every new one by using 3 Shards from Chests (or buying 3 mysterious Champs 5100IP). It's recommended to wait a little after the release to avoid bugs, though.

On the other hand... being IP efficient isn't really important once you own all Champions. =P