I have difficulty thinking in metrics but using a gallon jug is really a good way to keep track.
So three liters is equal to 0.792516156 US gallons.
Doctors may say to drink 8 glasses of water a day but how many ounces should each full glass be? One cup or two cups? 1 cup equals 8 ounces. 1 US liquid gallon equals 128 US fluid ounces. So there are 16 cups to a gallon. I use a quart jar on my kitchen counter. 4 quarts equal one gallon.
Obviously one could count every glass they drank, then as an 82 year old person always lose count. The same goes for quarts, liters, pints, losing count is the problem, not the measurement of how much to drink.