I found it a bit odd that Violet would be included as well, because I never thought they would reuse a character from one series in a 3 pack from another.
I don't think they will hold any character "hostage", though - while yes this product is designed to part us with as much money as possible, with the exception of the TRU disc packs not being marked properly, I feel like they have been as "fair" as possible in a game designed like this.
So, if you buy everything now, I don't think later there will be a 3-pack of, say, Barbosa, Davy Jones, and a new 3rd character that can't be purchased as a single.
At least I hope - haha - but I do think they won't pull anything like that, we are all gladly spending enough as it is.