How 3mG can be dangerous if the Earth's magnetic field is 500mG?
It is all about frequency. The 3 milliGauss limit is referring to high-frequency fields. High-frequency magnetic fields induce electric fields, which cause currents in human tissue. These currents cause heating, which can be dangerous at levels that can be achieved by cell phones and other equipment without proper management and testing. Field limits should depend upon exposure time and frequency.
Static magnetic fields up to tens of thousands of Gauss are not known to have harmful physiological effects, though there are people that worry that they do. Common clinical MRI machines, for example use static fields of 15,000 Gauss and 30,000 Gauss, which is 30,000 and 60,000 times the strength of the Earth's magnetic field.