I feel like so many people are missing the point of the third episode, so let me throw my two cents in.
The point wasn't to make a love triangle, because see how quickly it ended? Tony's relationship with Nicole was introduced AND wrapped up within 3/4 of an episode. The point of having Tony dating someone else was for Ziva to have her "oh shit" moment. You see it happen when he's talking to her on the couch about how he wanted to settle down and commit now that Tali's getting older. The hurt and sadness she feels about it is her proof that she's still in love with him.
The whole episode is, actually, about Ziva's feelings. the flashbacks to her and Tony making love happens in her head. And then there's Nicole calling her out for having unresolved feelings which she doesn't deny.
And all of this is designed to bring us to the penultimate scene. This moment here is the entire point of the episode. Ziva tells Dr Lang that "there 's never been anything normal about my relationship with Tony" to which Dr. Lang replies: "A good relationship meets your needs. Whether or not it follows societal norms is secondary to finding a partner that is compatible with who you are and what you want."
This is the moment Ziva knows that she's truly ready for a real relationship with him – that she's better equipped to handle it. She knows in this moment that they both want the same things; that he's everything she wants and needs in a partner. "It's okay to want to be happy." This isn't just her repeating Dr Lang's words back to Tony. It's her allowing herself to choose happiness, and knowing that her happiness is with Tony. Alternatively, this episode was a way of demonstrating, as John McNamara said, that Tony has been sleepwalking through his life ever since he and Ziva broke up. And I think the next episode – 1x04, Wedding Crashers – will prove to be Tony's own "oh shit" moment in the way this episode was for Ziva, during the moment he sees her in the wedding dress and watches her fake marry Boris.
He realises that all this time, he's been denying how in love with her he still is, and how much he still wants to be with her. And seeing her in that dress is when those feelings hit him like a freight train. In the way that it took Ziva nearly losing him to Nicole for her to realise that Tony's her future, it takes Ziva walking down the aisle (albeit undercover), for Tony to realise that Ziva is his. It shows both of them that there is nobody else except each other. So, no, this episode wasn't designed to create a love triangle to push them apart, but in fact, the opposite. It was designed to push them back together.
