In Game of Thrones S2E03 Tyrion tells 3 different stories to The Spider, Littlefinger and the Grand maester in-addition specifically stating that the Queen must not know. We learn it's a ploy to find out who is loyal to him and for him not to suffer the same fate as Eddard Stark but what intrigued me the most is if any of the stories he mentioned is true, Are those theories are completely made up or did he put those plans in motion at that point?
Well, he told Grandmaester Pycelle that he will marry Myrcella into Dorne, which is what Pycelle tells Cersei and in fact also what Tyrion insists on doing once Cersei finds it out from Pycelle (and what ultimately happens thereafter). So the story told to the untrustful of the 3 persons in fact turns out to be the truth. But it is still unclear if that was what Tyrion planned all along. What is clear though, is that he always planned to marry Myrcella to another house, which is evident from his strong conviction of following through with that plan, on the one hand to get her out of King's Landing and on the other hand to consilidate the Lannisters with that house.
But Tyrion is quite a strategic thinker and pretty much all of those three different wedding plans for Myrcella sounded equally good and it's probably irrelevant if she bonds with Greyjoy, Dorne or Arryn. So I would even go as far as to say that he probably didn't care which of the three plans are set in motion and he might simply have planned to choose the one that gets told to Cersei. This strategy would also provide him with the most backing, since changing his mind about the specific house once Cersei learns the (supposedly) wrong one from her spy might have undermined his point and might have made his plan look more like a staging than a serious strategy.
In the books, it was by luck that Tyrion's real plan was the one that was told to Cersei. Here is how that happened: - Tyrion wrote a letter to Doran Martell, promised to marry Myrcella to Trystan Martell, then gave it to Pycelle to deliver. Pycelle secretly read the letter, then sent it to Dorne. Because the letter was in fact sent out, it must be Tyrion's real plan. - After that, Tyrion told Little Finger that he planned to marry Myrcella to Robert Arryn. - After that, Tyrion told Varys that he planned to send Tommen to Dorne as a ward. Here is the book chapter that it happened: http://towerofthehand.com/books/102/018/index.html