please

You can only get oaks letter from nintendo events eg.TOYS R US
Apart from that Warrior13 answered your question.

Here are the answers:

1. To find Cresselia, you must use your Making Map to track Her down; which you receive the Marking Map from the owner of the Poketch Company.

2. To find Shaymin, you must somehow receive Oak's Letter from a Poke Event, then you must bring the letter to Professor Oak; which he is standing by the big boulder, at the end of Route 224. After you give him the letter, I beleive you must touch the rock, then shaymin will appear, then it will run away, you must follow it to Flower Paradise, and after you catch up with it, you can battle it.

3. To get Regigigas to awaken, you must migrate Registeel, Regice, and Regirock from a past version, then have them in your party, as you go to Regigigas and he will awaken.

Hope this helps you!!!

You will be able to follow Cresselia with the Poketch app Marking Map, it will show a small circle, and that will be Cresselia. Unfortunately, yes, you have to go to an event to obtain Shaymin. Or the alternative is too cheat and use an Action Replay.

Finally, for Regigigas you need to bring Registeel, Regirock and Regice to Regigigas and it will awaken. The only way to obtain the three Regi's is too migrate them from a past version.

1.After confronting Cresselia,you can go to any route but it's like a game of hide-and-seek with Mesprit but Cresselia is faster.

2.In order to get Shaymin,you must go to a Nintendo event that features Shaymin,or use a cheat in the japanese version,or use the Action Replay.

3.Regigigas can be awakened by migrating or trading the GTS for a Regirock,Regice,and Registeel. Go to where Regigigas is and then battle it.

Hope this helps you out.