They represent gas, liquid and solid. Giratina's ghost typing being gas.
No they don't. They represent space, time, and other deminsion.
No they don't. They represent turkey, ham, and cheese.
Idk if you're dumb or trolling but how would sandwich condiments be what they represent? What the hell? I don't see how any of these 3 pokemon fit the criteria. Giratina is cheese? And palkia is a turkey?
This is what my friend Professor Wormwood said when asked this question (though the question was specifically about Palkia):
"It's sort of difficult to think about, but consider it to be a fail-safe. Although Palkia is designed to be the source of all space, it must also obey the rules set down by time in order to exist; the same goes with Dialga. As such, their types cross-reference each other: Water is indicative of the flow of time, while Steel is indicative of the concreteness of space. This keeps the two forces in balance with both one another and themselves."
http://mutitus.deviantart.com/art/Palkia-Report-160255940 (Look in the comments)