SMITE

It's as easy as firing off your Ultimate and running into groups of enemies, making sure to let your team-mates finish the enemies off. If you want to get this done quickly, play Arena and max your Cool Down so you can use your Ult more often.

I was attempting to "hit at least three enemies at once with the Ultimate and hoping they would get killed off by team-mates within five seconds" - which seems fairly difficult. However, it would seem as though they can be separate instances of just getting one assist per Ultimate.

Also, XBL/TA doesn't seem to track the progress which gives the impression it might have to be done in one match. On the contrary, and likely due to SMITE's glitchy achievements, I do believe this can be done over multiple games as this achievement popped for as I was 0K/0D/2A shortly after starting my second game of attempting this and only Ult'ing once.

I just obtained this achievement while playing conquest mode as a support Xing Tian in the duo lane. I bought the active item "blink" so I could surprise the two enemies in lane at once and grab them with the ult right away.

You must get the assist within five seconds of using an ultimate, but the three assists total can be spaced out over many ultimates over the course of a match. I was lucky enough to pick up two assists after using blink-ultimate the first time, and then picked up the third assist after an ulti at a later point in the match.