Not trying to discourage you, but your Pokemon, if 100% legit is quite rare, given that you transferred it from any of the Sinnoh games, caught through the event. However, nowadays, people use hacks and other prpgrams like ActionReplay and Pokegen, to get a desired Pokemon. This of course is illegitimate, but wide spread, as these are received by unsuspecting people on the GTS and the Wonder Trade.
Anyway, the standard rarity of a Pokémon is 1\8192 for all Pokemon, and I think Arceus is no exception ( unless it was shiny locked, which I doubt it was).
Now about the pther two, sorry but they're more common than the Arceus, what with the DexNav thing, the Masuda Metod, and Shiny Charm. Regardless, they're quite rare, ar around 1/4096 chance if breeded by Pokémon of different countries, and nearly thrice as likely with the Shiny Charm.
Still, quite a feat, and if they're legit and\or not cloned, then you're one lucky person!