Choose to "Practise With Bots" and set bot difficulty to "Easy". Choose game mode "Casual", and map "Dust II".
Choose to play on team Terrorist. Buy "HE Grenade" and "Molotov Cocktail".
Immediately run to "Middle" and throw Molotov Cocktail on the ground in front of "Mid Doors", where most of the Counter-Terrorist team is probably already standing. Then follow up with the HE Grenade in the same spot, hopefully killing off 3 of the now fire damaged CTs, unlocking the achievement!
NOTE: This might take a couple of tries. Sometimes CT team moves unexpectedly, and sometimes you're unlucky with your throw. Either way, this has gotten me multiple x3 kills with the HE Grenade, including unlocking the "Three the Hard Way" achievement.