You have to have it as a .sgm or .sav for it to be possible (sav preferred). gen3 pkms aren't the same filesize as gen4 pkms, so you have three options:
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xboo cable with a retail/flashcart for gen3. Basically you just push a save file onto the cartridge and import on your NDS, assuming you have one.
I really doubt you would spend time doing this, but just putting it out there.
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DeSmuME import with the (rom+save of gen3) and a (rom+save of gen4)
I do this method for when I import my gen3 RNGs, as it is the more natural way of doing it.
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or use PokeGen in conjunction with Enciclopedia Pokemon to create pkms.
Depending on what gen3 game you have, you can use Enciclopedia Pokemon, which is a 3rd gen save game editor. For the english version go to Kazo's post (#5)
http://www.projectpokemon.org/forums/showthread.php?6683-GBA-to-NDS-pkm-files
From the information Enciclopedia Pokemon gives (PID/IV, level, exp, ball, pokemon etc) and knowing how to use PokeGen and what goes where, you can then run them through Trashbytes.exe to effectively pal park them.
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gotta keep those bros, eh?