While I personally attempted this and gave up days ago, I found a thread that gives very good advice for this tutorial that definitely would have helped me.
Struggled to do this for a long time too until I realized that I misunderstood what to do: 1) it says enemies, not machines. Try bandits because you can one hit them. 2)multiple enemies means two, so you need to kill three groups of 2+ enemies
You can either wait until you see bandits standing close to each other, activate the quest and take them out or do it as I did and run between bandits around until two of them are close, take them out, repeat. I did it in one single bandit camp that way.
I would suggest using a shock tripwire to stun multiple grazers, then lob the blast onto them to kill several at once :)
I completed this one by using corruption arrows on two striders, and leading them into each other, then lobbing the blast bomb once they had damaged each other a bit. The corruption effect ensured they stayed close together, as they wanted to fight each other. I had planned to use a tearblast arrow to damage them, but I didn't need to as they took about half health from each other in their first kick.
Any of these seem like extremely viable solutions. Since the tripwires were my favorite weapons by far, I'd suggest that one first, but I, too, misread the objective and understood it to only be machine kills.
If the shock tripwires don't do it for you, I'd suggest that one next.