SMITE

As Yu Huang, damage three enemy gods with the explosion damage from Dueling Dragons.

Duelling Dragons is Yu Huang’s ultimate ability.
When used, Yu Huang manifests a huge surge of dragon-shaped energy, becoming CC immune and summoning the Pearl Dragon to oppose him. He then has 3s to aim his dragon of energy toward the Pearl Dragon, who will charge directly at it. Both dragons damage and push enemies caught in the blast. If the dragons collide, a damaging explosion occurs in the area. If Yu Huang is Attuned, the width of the dragons is increased by 50%.

For this achievement, you must hit three enemy gods with the explosion portion of the ultimate, where the two dragons collide. I presume this must be three gods in a single explosion, but the achievement doesn’t specifically state this.

This is best attempted in Arena or Assault, as walls (like those in conquest) can prevent the two dragons from meeting and causing the explosion in the first case.

You can make this a little easier on yourself by using Yu Huang’s passive, Master of the Dao. You gain a stack every 15 seconds, then additional stacks any time you damage an enemy after 3 seconds of neither taking or giving damage. When fully stacked, you become attuned and your ult will be come wider, which will push more enemies into the explosion area.