My rabbit has drunk less and less as the days have progressed and I'm wondering what's going on. She used to drink a whole bottle in a day but now she's down to 4 days per quart of water. What should I do?
First the disclaimer, this is a potential health issue that you should discuss with your vet during your annual checkup. As the change has occurred slowly over 3 years it is probably not worthy of immediate attention, but if you notice in sudden changes, or water intake decreases beyond current levels, contact your vet.
We often see rescued rabbits having high water intake when they are first rescued. A quart of water is a lot for a rabbit. Often they were not well cared for and will drink to excess, this usually slows down overtime, much as you have described. A quart of water lasting a rabbit 4 days is on the low end of water intake but not inherently bad. In particular if your rabbit is getting good quantities of high fluid vegetables like lettuce or fresh grass, there is little cause of concern with this level of fluid intake.
A larger concern is high fluid intake, rabbits are more susceptible to polyuria/polydipsia which is a symptom of several kidney diseases. I have been told by more than one rabbit specialist that excessive drinking in rabbits can damage the kidney, in some cases rabbits need to have the water intake restricted. I am not remember the word for this currently and a good reference about it is not jumping out at me.
I assume your are actually changing the water daily or every other day, and cleaning the bottle and nipple regularly, to help prevent water borne diseases.
Wire bottom cages, can give false impressions of increased water usage, if the water nipple leaks it may not leave a trace. Rabbits do drink from bowls, these tend not to leak and are easier to clean then nipples on water bottles.
In summary, keep on eye on her water consumption, her high water intake when you got her is more worrisome then her low intake now. Changes in temperature and diet can also impact water intake, take those it to consideration as well.