At the end of scene 3, Iago realizes that he has a great weapon with which he can bring down Othello, namely Othello's trust in him.  So he determines that he will use this to destroy him, and he will use it in regards to Desdemona.

Because Desdemona is a child of royalty, he thinks that she will tire of her new love just as she will lose her appeal to him after some time.  So he thinks that he should be able to bring his evil ambitions to life by using their relationship to destroy Othello.

He also encourages Roderigo with this knowledge, suggesting that Desdemona will be his as soon as their relationship sours.