MegaGargomon.
Yes, you can get MegaGargomon. And you don't get Terriermon, so don't try searching for his DRI. Anyway.
Train Agumon to level 40 so he learns WarGreymon and train WarGreymon to level 20 to learn MetalMamemon.
Train MetalMamemon to level 40 to learn MetalGarurumon and train MetalGarurumon to level 90. Then, he will learn MegaGargomon. So, it's possible to get all the Mega Digivolutions from session 3 with just an Agumon.
Off topic: If you must know why I only explain these digivolutions with Agumon it's because he's my strongest in my game (Level 62) and knows most of the digivolutions.
If you need more help, we've got more Digimon World 3 cheats and also check out all of the answers for this game
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