Processor Number i7-3517U
# of Cores 2
# of Threads 4
Clock Speed 1.9 GHz
Max Turbo Frequency 3 GHz
Like is says, 3.0 it's the max turbo frequency it will gain when on heavy load, very likely it's hitting the 3.0Ghz when you're gaming or doing video rendering etc.
If you wanna make sure it's reaching the 3Ghz freq, run a prime95 stress test and monitor the frequency with CPU-Z or any other similar program.