If the GPS 3 looses signal, turn off the other GPS to see if that cures the problem or try flying in the same areas using just the GPS 3.
I had this problem with using a GPS 3 and a pannel mounted (old) GPS. The GPS 3 even had problems with the antenna extension.
The answer which was given to me was that the internal clock of the other GPS was radiating a signal which interfered with the GPS 3.
As long as I keep the pannel mounted GPS off, there are no problems with the GPS 3.
Regards,
DFC