I need to meassure 3 differential (not three floating) signals with a NI USB 6366 & BNC2110 ...
in the FS setting it seems that all negative inputs are connected to the same AGND via 5k and 100nF building a asymmetric load to my sources. 
However since I don't use AO (and the 6366 measure differential anyway) it should be possible to measure differential in the GS (ground referenced) mode and use two 1M resistors per channel as bias drains to AGND, rigth?
That way I should have a symmetric load.