In my opinion, a Naive nature for Dragalge, so it takes down his down his sp defense, and boosts his speed (As it is the lowest)
For Groudon, I'd say a Timid or Calm nature to bring down his attack and boost his speed or his sp defense.
And for Aegislash, you would have to choose. I would recommend something to take down his sp attack because most of his moves are physical (I think :/ )
Check the natures here www.pokemondb.net/mechanics/natures
I hope this helps, these are just my opinions :D