1. Dialga: This time-controlling dragon is found in Spear Pillar, which you can access after defeating the Elite Four.
2. Palkia: This space-controlling dragon is also found in Spear Pillar, and you'll encounter it after capturing Dialga.
There is no third legendary Pokemon in Pokemon Diamond, but Giratina is featured in the sequel, Pokemon Pearl. While Giratina can be found in Diamond, it requires a special event to capture it, and it is not part of the main story.