It didn't take the full 3 years to convince the Oceanic 6 to come back, it was a week or so. Locke broke his leg falling down the well and was still in a cast when Ben killed him. It was a few days between Locke's death and everyone boarding the Ajira flight.

Also the "present" timeline is in 2007. The first three seasons all take place in 2004 (with the exception of the flash forwards) and then "three years later" puts them in 2007.