There is no best computer for $3000. That's because computers need to be designed for specific tasks. A computer that plays games really well is way different than a computer that serves files to a large office, or a computer designed for an architect.
For three grand you can build just about any computer. But Proximon is correct. You build a PC for a specific purpose. Otherwise if you come to the computer world with 3 grand and just wanna spend it, you get a low end mac.