2 of them are gotten from defeating 2 of the 4 henchmen (can't remember which one) of Maira. And you're gonna have to beat them anyways in the game.

The 3rd is from defeating the monkey king that you'll get to later on and it is a side quest.

Where is the monkey king?

Book IV-Goddess Maira: Darkness. Before heading to Remoria Continent. Bring Leona w/ u,
repeat, very important. U'll see. Go to Roma City. Head out of town towards Pasela. u will
see that the door of the fence has been opened 4 u to get the treasure chest inside. Climb
up the ladder. Head SW along the train track into the tunnel. Get the treasure chest along
the way. Out of the tunnel, u will see the trail blocked by huge rock. Step off the trail.
Scene starts. Defeat the monkey. Follow the monkeys. U will be captured, thrown into jail,
have conversation w/ their leader. Mission: Defeat the tyranny monkey Kaiser. Rest/Save w/
one of the monkeys there. Head out of the hut. Explore the huts along the way to get
treasure chests. Eventually, meet the Monkey King. Defeat it & u will get the 3rd dragon
orb.

I tried to kill the monkey king but he refuse to die.

sorry but i don't bring leona, and just step out until the monkey king and when he died, i
just got the dragon orb