As this achievement description has changed, and the solution by Immortal x Dead is no longer correct, I thought I'd add a new solution.
This award now states;
"As Ymir, kill three enemy gods at once with your ultimate."
Ymir's ultimate is Shards of Ice. When activated, a large circle will appear around you and ice will slowly spread from when you're standing to the outside of the circle. It takes 3 seconds to reach the outside ring. It applies a 30% slow to all enemies who are trapped on the ice before the explosion.
This ability can inflict massive damage on unfortunate enemies, however only if you allow the ice to reach the outside edge. You can choose to detonate the ability early, although it does reduced damage depending on how soon you detonate it.
Enemies with movement abilities can jump out of the area, as can those that have beads or an ability that makes them crowd control (CC) immune, as the slow is removed. This achievement is awkward to get, however will come with time. I've played Ymir a little and have twice got close to getting this achievement.
The blink relic is basically essential, allowing you to jump into a group of enemies. Your other abilities are also key here. Use blink to jump towards grouped up enemies, then either use your Ice Wall to block enemies in, or your frost breath to freeze enemies in place for a second or two, or both. Once they're caught in place, immediately use you ultimate to start freezing up the area, as long at least 3 gods are caught in it. Remember, the longer you're charging the ult, the stronger it is, up to 3 seconds. Explode it when you think the time is right and you should pick up a triple kill.
While Ymir is a guardian, and building standard items still allows you to unlock it (my two closest attempts at this were while built as a guardian), you can also build hybrid or outright damage, making it easier for you to secure kills, but sacrificing your survivability.
Items such as Gem of Isolation (which increases the slow on your ult) and Soul Reaver would help.
You'll also have to watch out for allies taking your kills and enemies collapsing on you (as you're rooted in one spot when using your ult).