RTSI is the best way to synchronize multiple DAQ devices.
You can use one single RTSI cable to connect all three DAQ devices together. The way to code that is also simple, you can call Route Signal.vi, if in Labview, or Select Signal, if using NI-DAQ function calls. You will need to have multiple calls of those functions, in a way that you route the signal from the master board to a specific RTSI line, and then to have two more calls, to catch and route the same RTSI line you used before, to the signals of interest on the other two boards.
Hope this helps!