The final two Legends, but actually both Mythcial Pokemon, of the Third Generation are Jirachi and Deoxys. These two Pokemon also created a new trend in the Pokemon Company, as there had never been two Mythical Pokemon apart of the same region until that time.
Jirachi is the Wish Maker Pokemon, awakening every milienium to grant wishes. It was the third installment of a Mythical Pokemon to have straight 100 Bases for each of its stat, like Mew and Celebi before it. It too has smiliar shape and size to these Pokemon, as well also receiving its own movie about it.
Deoxys, however, is a Mythical Pokemon unlike any one before or since, mainly due to having four different forms (Normal, Attack, Defense, Speed). It was once a virus in space before becoming a Pokemon. Deoxys is also the only Mythical Pokemon that can be obtained in-game without an event, as well as having a connection to famous King AZ and rivalry to Rayquaza.
Jirachi can be used in a number of ways competitively like Mew and Celebi, but is usually used as a speedy, offensive sweeper than loves to make Pokemon flinch with its ability Serene Grace and moves like Zen Headbutt and Iron Head.
The good thing about Deoxys is that you can plug in any of its forms to better fit your team. Normal Deoxys is a speedy, offensive nightmare; Speed Form is fast, but lacks power and defense; Attack Form has more than enough power, but is lacking desperately in defensive abilities; and Defensive Form can take hit after hit, but isn't that fast of a Pokemon.
#385 - Jirachi: Being that it is a Mythical Pokemon, unfortunately, Jirachi has only been available by Pokemon Events. This reigns true to this very day.
#386 - Deoxys: The same can be said about Deoxys, well, at least it should be. Deoxys actually can be found in Emerald, Fire Red, and Leaf Green on Birth Island if you received the Event Ticket to get to the island. But what makes Deoxys such a unique Mthical pokemon is that it is now actually available in-game without an event, as it can be caught in the Delta Episode in both Omega Ruby and Alpha Sapphire on/in Sky Pillar / Space. Pretty cool.