I believe that requiring testing every 3 years would help refresh drivers on the rules and regulations of the road. This could make for safer driving conditions and could possibly lower the amount of accidents.
Having to take a test every three years will put of young people who want to drive. This is due to the fact that tests put you under pressure, and add to the stress of things such as work, children, school. I understand where you are coming from in saying that if refreshes the memory of drivers, but don't you think that every day that a driver drives his car, he is refreshing his memory? A driver is constantly reminding himself on rules of driving (of course this isn't every driver).