Sounds to me like you need to use the "greedy algorithm" which is just a fancy way of saying that each time you have a choice, you take the best remaining. Here, you just want to get "largest possible total Litres" without regard to what is in the container. Assuming you get to go first, you should take the 3 L water bottle. The other three people, assuming they are following the the same "algorithm", will choose 2.75 L, 2.5 L, and 2.25 L water bottles. That leaves the 2L milk and 2L water bottles for you. Take either one of them. The other three people will take the other 2 L bottle, and the two 1.5 L bottles. Continue always taking the largest remaining bottle.
(If the other three people do NOT follow this algorithm, you still take the largest bottle remaining. You will just do better than if they followed the "greedy algorithm" also.)