The blast killed thousands. How many thousand? Well, we see 4 burning corpses in the first shot.
So 4000.
We see 3 different dragons. 4000 times 3 = 12000
Divide that by the number of people seen in the last shot, which is 13, minus the Inquisitor. So we get 12000 divided by 12 = 1000.
Cassandra has 1 scar, as well as Varric. As they are the only companions talking, that counts. 1000/2 = 500
The Inquisitor kills 2 spiders. 500/2 = 250
We see 3 types of demons, and 2 Behemoths. Which adds up to 5. 250/5 = 50.
In the shot where the party runs towards the edge of a cliff, we see 20 crows fly away.
50/20 = 2.5
As we need a whole number, let us round off 2.5 That makes 3.
Half-Life 3 confirmed, ladies and gentlemen.