AMD's processors are slower on frequency. Everyone thinks that the higher the frequency the better the processor, so in AMD's mind it's giving you a number that's comparable to Intel. In other words, AMD's AMD64 3000+ is comparable to an Intel P4 that runs at 3.0Ghz - but it only runs at 1.8Ghz stock.
You were only wrong in the sense that you thought you'd have a "3 GHz PC", but, it wil perform about the same as if it were a fine running Pentium 4 3 Ghz processor, which is not a bad thing.