To get the three fields and Ho-oh
After you complete the main story somewhere you will find a Medicham hanging around close to the guild, he will direct you towards Lombre that is in the center of Pokemon Square. It will tell you about the Solar Cave and then you visit Whiscash, once that dungeon is opened up to you, you need to go several explorations(Rescue missions) in it.
Once you have done those explorations(Rescue Missions) then a Spinda will visit you in the guild and give you a clear Wing. Head to the Great Canyon and talk to Xatu who will tell you what it's used for, and mention the three fields. Once you do that the first field will open which is Fiery Field.
At Fiery field you will meet Entei, once you make it though that field the Clear Wing will turn Red and the next field Lightning field will be opened, head to that one and meet Rakiou, once you do that the Clear Wing which is Red will turn Yellow, the final field will then open.
Meet Suicune at the end of that Dungeon and then the Wing will turn into a Sunset Wing which will open up Mt. Faraway dungeon which is were the legendary Pokemon Ho-oh is.
If you need more help, we've got more Pokemon Mystery Dungeon: Blue Rescue Team cheats and also check out all of the answers for this game
Comments for this hint