Bagon would be a pretty rare(might be much more rare than dratini)as it evolves into shelgon furthermore to salamance.
Beldum will be about the same rarity as of course it evolves into metang and than into metagross.
Slacoth might be very rare because it's full evolution is slaking(5000 over cp)
Aron will be rare as it is the TOP 10 most strongest Gen 3 Pokemon
If you have any more ideas feel free to comment down