You can't, only in Platinum... Shadowscar I caught them.
you have to catch Palkia(in pearl)orDialga(in diamond)
You go to the three caves and catch them.
no
no.
try to catch your legendary. It varies on which legendary depending on what starter you chose.
at the midle of the three lakes at the midle of the three lakes
a utra ball or a master ball
One each.
yes you can also catch them in soul silver.
Mespirit, Uxie, and Azelf. i don't know if these are th 3 legendary Pokemon you wanted, you have to be more specific
there are caverns in the middle of the three lakes the three spirits appear there after you beat the Pokemon league.
grovile