The amount of your instalments is calculated on the basis of consumption in your most recent period. The three quarterly instalments are intended to cover 3/4 of your next adjustment bill. This means that when you receive that bill, only 1/4 of the amount remains to be paid.
However, if there is a change in your consumption habits (change in household composition, construction of a swimming pool or pond, etc.), you can adjust the amount of your instalments to suit you, avoiding unpleasant surprises.