This Achievement can be quite difficult in the early game but gets much simpler later on. As the achievement describes, you must have a total of 3 unused Zybellium. This is done by completing 3 compression events (or 2 with the free perk "Zybellium Mastery") without spending any you have collected. The easiest way to get this achievement is to wait until you have unlocked "Zybellium Mastery" and put any "Prestige Points" into perks that increase your damage. I would recommend not activating the perk "Bigger Pile" when attempting this achievement.