Will a 2003 Buick regal 3.8 engine fit 2000 SS Monte Carlo?

No, a 2003 Buick Regal 3.8L engine will not be a direct bolt-in replacement for a 2000 SS Monte Carlo's engine. While both are 3.8L V6s, there are significant differences in the engine's mounting points, accessories, and possibly even the block itself depending on the specific 3.8L variant in the Regal. You'd be facing a major and likely impractical engine swap project requiring extensive fabrication and modification.