If Age of History 3 generated the same gross revenue on the Epic Games Store, Łukasz Jakowski would keep approximately $3,272,261 - that's $764,781 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $309,854. 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 $2,507,480.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.