If Sniper Elite 3 generated the same gross revenue on the Epic Games Store, Rebellion would keep approximately $11,803,305 - that's $2,345,943 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $1,473,178. 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 $9,293,538.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.