Here is a little info from Jeff Foster.
On the V3, the multi-frequency operation can utilize either one or two ground-balance points. Standard 3-Freq operation ground-balances to only one iron-based mineralization region. "Salt Compensate" is the name of the 3-Freq mode that ground-balances to salt and iron mineralization simultaneously.
On the DFX, the 2-Freq modes always have the two ground-balance points. There are trade-offs for doing this. That is why it's beneficial to understand how and when to apply the Single Freq modes on the DFX. On the V3, there are still benefits to using the single Freq modes, but they are less important.
In terms of "noise cancel" capability, the V3 has a couple features. First, the "Live Controls" are Live, so you can tweak the Rx Gain and Sensitivities with the unit in search mode to obtain stable operation. In addition, the Freq Offset also can help in some instances (fitting best between 60Hz vs 50Hz power grid harmonics was one design intent for the Freq Offset feature, although it may help in other situations).
Enjoy.
Jeff Foster