XCOM 2

The Spark has only one ability pertinent to getting this achievement (well, not including the default Overdrive): the Adaptive Aim ability unlocked at Aspirant rank - the Spark's equivalent of Corporal. With Adaptive Aim, each shot the Spark takes in the same turn will have the same aim chance, but without it, your aim goes down by 15% with each shot. So, you could start off with 100% aim, but it would be down to 85% on your second shot and 70% on your third. And as we all know, a 70% shot in XCOM is usually a miss. So, plan on getting the Adaptive Aim ability before attempting this achievement.

Like other soldier classes, the Spark's aim increases slightly with each promotion, but unlike other classes, there are no new weapons or mods you can equip to increase either its aim or ammo capacity. I actually recommend that you try this with the Spark's lowest-tier autocannon because it does the least damage. If you get a juicy, high-percentage target, you don't want the first two hits to kill it.

You will help yourself by taking along a grenadier at Lieutenant rank or higher with the Holo-Targeting ability. Ideally, you will find a single, very durable enemy that the grenadier can holo-target first and then the Spark can engage Overdrive and shoot three times for a hit. I did it with a heavy turret. It helped that my grenadier missed her holo-targeting shot, thereby leaving the turret able to withstand two hits from my Spark. (Another way to go about that would be to use Suppression to apply holo-targeting, rather than an aimed shot.)

The achievement does not require that you get all three hits on the same target; that way just helps you take advantage of holo-targeting, But by all means, if you kill the target in the first or second shot, try shooting at another enemy.

The probability of getting three hits in a row is the base chance^3 power, so having a high base chance is really important. Your probability of getting three 85 percent hits in a row, for example, is 61 percent, while your probability of getting three 90 percent hits in a row is 73 percent.

If you miss one of the shots, remember that Overdrive has a four-turn cooldown. Don't forget to reload on the turn before you intend to try for the achievement.

I assume that you could use turn-extending abilities like the PSI's Inspire or the Distraction perk you can sometimes get from hacking lampposts to help with this achivement, but I didn't test this.

EDIT: With the War of the Chosen expansion, Sparks' weapons can be modified (expanded magazine, scope, etc.), making this achievement easier to get. Not to mention the free actions given by headshotting The Lost. You may end up unlocking the achievement without even engaging Overdrive.

To add to the other solution you can save scum this. Get in range of one or more targets that can withstand your shots, overdrive in the next turn, save, try shooting the first one and if it hits save. If it doesn't reload and you must do something with another unit and try again. After another successful hit save an so on.

This game, like its predecessors, calculates a set number of RGNs at the beginning of the turn that are applied in the same order. Knowing this is very easy to control the hit chance and after the third one the achievement is yours.