If Warhammer 40,000: Dawn of War III generated the same gross revenue on the Epic Games Store, Relic Entertainment, Feral Interactive (Mac), Feral Interactive (Linux) would keep approximately $21,950,340 - that's $3,844,937 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $2,856,864. 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 $17,365,043.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.