Looking for 3 Assassins..

[Pirate Rose]

Hmm, ok. But it's not a storyline quest. You should be able to turn in all your storyline quests without having gotten your assassin. You can carry that quest forever without it affecting your storyline quests. And yeah if a quest isn't completed, you can't turn it in until it is. Since you have 2 of 3, you can't turn it in.