Question

Can I get legendaries from other generations in platinum

Answer 1

yes once you beat the elite 4 you talk to oak in eterna city he will say that the 3 legendary birds have been spotted near sinoh and they will be roaming
they are lv 60

if you got the Regigigas from the 11th movie and transferred it to platinum
take it in your party to Mt. Coronet, Route 228, and Iron Island you will find the Regis where the caves were with the metal coat, nevermeltice, and hard stone

Answer 2

Yes. There is the legendary bird trio. With events, you can get a bunch more.

Hope this helps

Answer 3

The legendary birds are roaming, The Legendary Golems and Dogs/Beasts are events, etc.

Also, you can migrate from Gen III to Gen IV.

In short: Yes!