If Fallout 3 generated the same gross revenue on the Epic Games Store, Bethesda Game Studios would keep approximately $4,975,274 - that's $1,113,124 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $542,083. Epic charges 0% on the first $1 million in annual per-app revenue and then 12% on the remaining amount. On GOG (30% cut), the developer would earn approximately $3,862,150.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.