With what chassis? In what language (LabVIEW? C?)?
If you're using a CompactDAQ chassis (such as the cDAQ-9178), then in DAQmx, if your NI 9225 has the device name cDAQ1Mod1, then you can pass the string cDAQ1Mod1/ai0:2 into your DAQmx Create Virtual Channel VI to acquire from all three channels (ai0, ai1, and ai2) simultaneously. See also: What is the Syntax for Physical Channel Strings in DAQmx?
