Compass Mag3X offset


I started to tune my arduboat the last days. Its equipped with a Pixhawk 2.1 cube and Here GPS.
When I look at the LogFile, there seems to be something strange. X of Mag1 and Mag2 is consistent, but Mag3 has a quite big offset. Is that normal?
And I´m wondering which stated Mag is which real device? 1&2 internal cube compass and 3 the external Here? Or 1 the Here and 2&3 the internal?

Thanks for any advice.