Gen 3 Glocks are the least likely Glocks I'm inclined to dispose of; mine (a G17, G19 and G21) are exceptionally reliable, well vetted, and more than acceptably accurate. And they have the older Tenifer undercoating and "frying pan" matte overcoat finish, which has holded up exceptionally well.
I don't doubt that there are quantifiable improvements with the Gen5 Glocks, as I'm currently experiencing with my Gen5 G23, but the Gen 3's are excellent guns in their own right.
Best, Jon