The device number in question is the device number of the DAQ board that is cabled to the SCXI chassis. I presume you have only one DAQ card cabled to your chassis, so, the device ID will be that of the board you reference in your question --- device ID=3.
Now, for some additional help, you can either use MAX and create a virtual channel name, or, you can perform a direct channel call. If you use a direct channel call, the channel call should be (ASCI String Characters)
ob0!sc1!md4!0
Which means: ob0 - on-board channel 0 of your DAQ card
sc1 - scxi chassis 1
md4 - module 4
0 - channel 0
I hope that this helps.