Hello BML
Have you considered you may have a leak in your water container even a hair fracture. Is the area around the container quite wet?
Also is it a plastic or metal drinker? Plastic drinkers can crack easier than metal ones obviously. What breed chickens are they as the size of the bird and the body mass can also affect how much they drink. for example if they are brahmas, their water intake is very different to if they are pekin bantams. This may sound obvious, but you wouldnt believe the amount of people that have asked me that only to find they were thinking of completely different breeds,
. sure thats not the case though.
Sure, for your amount of birds you would be looking at a maximumcapacity of 3 litres for 3 birds a day. So a cheaper option would be a 3 ltr plastic drinker, although if you got more birds in the future, you may want to get a larger one such as a 6 ltr or more.
Work on the basis of half a litre or 1 pint a day in average temperature. In hotter weather they will drink up to a litre a day. However they need fresh water every day.
Also I would go for a plastic one as they are easy to clean, can be used for additives to the water (ACV) or Verm-X and are fairly cheap for what they are. An average price for a 3 ltr plastic drinker would be around the £5 mark
HTH and good luck