how to get bagon, shelgon, of salemence

Well for Shelgon and Salamence you would need to evolve the Bagon or do a trade. As for the Bagon,you can migrate it to the pal park in your in ds if you have a Bagon or those other 2 pokemons in Ruby/Sapphire/Emerald if you have. Another thing is if you have wifi, you can do a GTS (Global Terminal System) trade and if you do have wifi and need trades specifically then use the following link below:

Http://forums.supercheats.com/forums.php?forum=1791

Hoped I helped.

Trade them or transfer them over from R/S/E.

Well, trades with friends or other people at the GTS, OR the pokemon modifier code. Please read it very carefully.
94000130 FDFF0000
62101D40 00000000
B2101D40 00000000
D9000000 00111D10
C0000000 0000000C
D6000000 000233E8
DC000000 00000004
D2000000 00000000
94000130 FFFB0000
62101D40 00000000
B2101D40 00000000
DA000000 00111D10
D4000000 00002400
D3000000 00000000
D7000000 020738D4
D2000000 00000000

I HAVE USED THIS CODE MANY TIMES, AND HONEST, IT DOES WORK. (: You put Bagon's, Shelgon's, and Salemence's NATIONAL pokedex number in the calculator app on your Poketch. Hold L, and run through grass. Only put one pokedex number in at a time.
Please keep in mind that you need Action Replay to do this,.