Technically saying, if these three were transported to Gen6, with the introduction of Fairy types, Dialga will become the best among them. Since Fairy types are good against Dragon, and Steel good against Fairy, Fairy-types will only do neutral damage to Dialga(while 2x against the others), and Dialga has some way to strike back.
Well if it was up to me I'd say it would be a tie. They're all so strong that each of them would be too weak to battle. So obviously none of them are stronger then the other. And in the first place why would they battle at random?
In Platinum, Palkia wins. Dialga and Palkia are the same level (and stronger then Giratina), but Dialga is Steel & Dragon, but Palkia is Water & Dragon. Dragon is even. Water beats Steel. So Palkia wins.
In all the odds are:
Giratina-50% chance of winning
Palkia- 40% chance of winning
Dialga-10% chance of winning
How did I get these percentages, let's just say I know someone in the Pokemon Company
Hope this helped!
He can teleport , change size , control gravity , send meteors , cause awesome waterbending (cause his space control ability can distort any matter) , turn anything to digital matter or warp spaces!
He can confuse or hallucinate those dragon brothers of him!
If you want an idiot donkey-like wuss with so-so stats compared to the others then pick dialga
If you want a cool looking draconian beast of a Pokemon that steem rolls practically everything, pick palkia
if you want a all around uber pick giratina origin form.
If you want a defensive behemoth pick giratina
Now I haved been thinkin for four days so I had saved it then chose one for a day chose another for a day then another and today on the 4thi thought a lot
dialga is good but has an advantage of roar of time in the darkrai 1 like arcues has roar of time but if you have seen arceus and the jewel of life giritina... Well this brings me to somewon ho said that garitina is weaker out its dimension but in arceus and the jewel of life it was out its dimension and would have killed dialga if it wernet for sheena so that puts dialga out.
Now palkia vs giritina very hard because I seen palkia do strong clever and amazing things but giritina has very special things it can do so I know who is best. Now if Palkia would simply fight with garitnagaritina would hurt palkia but palkia would simly grow bigga and create a like space focefield around garitina (I saw that in arceus and the jewel of life) around it for protection and when its ready let it go and do total destruction to giritina.
But some people might think garitina would easily break out but arcues as we know is the strongest had a bit of trouble breaking out than managed to as that means giritina could probbaly not break out. I coyuuld sit here and say there all wonderfull so I don't no hos best but I'm not going to I'm going to say that..... IT IS DEFANATLY PALKIA!
It is also because dialga is stronger and would win easily.
I once battled my friend with my lv.64 dialga and smashed his mewtwo,ho-oh,lugia and deoxys
I also beated the league with my dialga too