RVI PFI,
It does not appear that any examples with this exact functionality are available, but you should be able to use the Cont Generation - ExtClk.vi example as the foundation of your application. The master PXI-6713 will use the external clock signal as its update clock. Additionally, you will need to use Route Signal.vi to route both the mater PXI-6713's AO Start Trigger and AO Update signals to the RTSI (PXI Trigger) bus. The two slave PXI-6713s will both use these two RTSI lines as their start trigger (AO Trigger and Gate Config.vi) and update clock (AO Clock Config.vi), respectively.
Good luck with your application.
Spencer S.