With 3 year free agents, teams wouldn't be able to afford doing that. Who would pay Riley $20 million per year for 5 or 6 years and then sign several other players for even more? The owners costs would go way, way up to field a competitive team. Think a $600 million payroll. There would be multiple players for every position available. And the league would get even younger, as most teams would let free agents walk. The quality of the game would suffer.
Yeah, hitting free agency after 3 years isn't going to happen.