technically, from the time Buffy and Angel meet in 'Welcome to the Hellmouth' to the time he left in 'Graduation Day, PT2' is three years. but they didn't spend all of that time as a couple, concidering Angel went evil for half a year, and then went to hell for a few months.
plus, there was that time when Buffy was dating Scott Hope...so three years Angel was in Sunnydale, but they didn't spend three years as a couple.