If I'm understanding the question...
You need to beat Platinum, there's no need to worry about the Gen III game. You basically need a normal DS or DS Lite, where you put the Gen III cartridge into the GBA slot and Platinum in the DS slot. You go to Pal Park (which is only available post-game) in Platinum and start migrating your Pokemon!
However, it should be noted that you CANNOT migrate Pokemon with VisualBoyAdvance or No$GBA, it must be used with a hard copy versions of the games. Sorry.
Hope I helped. :)