Larry,
Are you using one DAQ device to control all three chassis? If you are, you will be unable to have two buffered analog input tasks running simultaneously. Since you DAQ device only has one scan clock and one channel clock, you will need to have all of your analog input channels from the first and third chassis in the same task. Because your second chassis does not contain any analog input modules, you are not experiencing problems with the tasks from the first and second chassis running simultaneously.
Good luck with your application.
Spencer S.