If Arma 3 generated the same gross revenue on the Epic Games Store, Bohemia Interactive would keep approximately $111,170,819 - that's $13,215,529 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $15,023,293. 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 $88,335,878.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.