Turns out, you can get the take a 3 with 3 achievement if you attack a 3 with 3 plus your commander. I feel like Fizzer should fix this, as it kinda takes away from the whole point of this achievement.

Good catch, I'll fix that. Thanks for reporting it.

How does one do it anyway?

Is it like a one-in-a-100 shot, and we just risk armies in different games until we get it?

3 on 3 is better odds than 1 in 100 I think, I got mine in a game without even intending to, he just happened to defend there and I got it.