Is drinking 3l of water too much?

Not if exercising...: Exercise and increased ambient heat will increase fluid requirements over and above the two liter, minimum threshold. Id you are engaged in high intensity sporting activities and become thirsty, you are finished, because you will lose more water than your GI tract can absorb: hence the rule, drink when not thirsty!

Yes: Yes but while you can get very ill from too much water, 3 liters will just likley be very uncomfortable if done in one sitting.