Drinking three or more cups of tea a day is as good for you as drinking plenty of water and may even have extra health benefits. The common belief is that tea dehydrates but actually tea does not only rehydrate you as well as water does, but it can also protect you against heart disease and some cancers.
SuDFB is partially correct in that tea does contain a few anti-oxidents and will provide plenty of water if it is drunk in abundance however there are still high levels of caffeine in tea and it does act as a diuretic which means that the water hydration level will not be sustained as you will need to go to the toilet more.