Why the first three groups in Anyder?

[TouchandFeel]

You can't fight 2 and then 2 because you have to kill the last group of the 3 to open the gate that separates out the last group before the boss and so the pulls are every group to the wall (first 3), then the last group, then the boss.

If you were to not take all 3 in the first pull it would end up being, one pull of 2 groups, another pull of 1 group to drop the gate and then one more pull to clear the last group before the boss. That is one extra pull.

Also since it is the first pull of the dungeon, tanks and the rest of the party have all their cooldowns available to burn, especially since the next pull is of only one group, so the tank can blow a lot of their defensives on that first 3-group pull and not worry as much about saving them for the next 1-group pull, leaving them to be back up by the boss.