They should remain the same rarity...but I mean, if you expand your world way past the third stronghold, it would be weird wouldnt it? That there are three strongholds within 2000 blocks of the spawn but none anywhere else in the entire world. Sounds a bit farfetched.
A more reasonable way to make it is every time a dungeon is about to be generated, generate a random number, so that 20% of the time, a stronghold generates instead of a dungeon. That way strongholds would appear all across the world, no matter how far from spawn.
Obviously the code would have to be tailored to match the seed number, so that the stronghold would be in the same place each generation. But that's fairly simple.