There's more than one type of Mega, so I don't know which phone you have. But to be honest, I doubt it. The two models have different capacity batteries, both significantly higher than the S3. Since the capacity of a battery is determined by the volume of electrolyte (assuming same chemistry), that means the volumes of the stock batteries will be different, so it's very unlikely that they have the same length x width, have contacts in exactly the same place.
It's hard to find specs of batteries (they quote capacities, but not sizes so easily), but I wouldn't bet my money on it.