Assuming 3 pints of 5% beer/lager then that is about 8.5 units of alcohol. Your body gets rid of about 1 unit per hour so to be alcohol free you should wait about 8.5 hours after your first drink assuming that you drank steadily and it's also dependent on lots of other factors (body mass, what you have eaten etc.)
Obviously you are allowedl to drive with some alcohol in your system but I'm not sure how to work this out.
a lot of that also depends on yo9ur body weight and if you are used to drinking that type of alcohol , also if you drank on a empty stoamch is abig effect