trois litres d'eau or trois litres de l'eau? which is correct and why, I am so confused on this

It should be trois litres d'eau.

The rule is -- in a nutshell -- that whenever you specify a quantity of something, you drop the article and only have "de". Note, though, that "nothing" is also considered a specific quantity.

J'ai du lait dans le frigo. -- I have some milk in the fridge. Je n'ai plus de lait dans le frigo. -- I have no mor milk in the fridge.

That makes sense to me. Thank you so much.