after I finished the Pokemon league in platinum..prof. oak told me about the 3 legendary birds..if I catch the 3,will oak give me a reward???
As simple as that. Though I suppose your reward is the bragging rights that you caught the legendary birds <3 and the ability to use them in battles. Legendaries are awesome after all :D
Source: Capturing them myself so experience
Yes you get the upgrade when you catch the birds
Nnnnnnnnope. You just catch them and you keep them for the whole sake of it. :3
Hope I helped. :3