Thanks.  Is there only one 3m cable permitted per stack, or can there be two 3m?

(e.g.: switch --1m-- switch -----3m----- switch --1m-- switch ......3m back to first switch......)

15 M cable will not work for stacking.

You can use as many 3m cables as you need.  There is no limit.

This is information for anyone else who is doing as I have been and Googling "Dell Longest Stack Distance".

I recently called and asked Dell's tech support about this; I was passed up 4 levels of tech support to someone who told me they had seen stacking work with 5m cables (15 feet).  However, I needed to connect two stacks that are 50 feet apart.  They told me they'd ask the engineers whether it was possible to get a stack working that far away with a slower link speed, but I never heard back.  It is noteworthy that 10G CX4 is specified to work up to a distance of 15 m (49 ft), but I didn't want to purchase an expensive 15m cable that it might turn out not to be usable.  Maybe if you used the CX4 uplink module (GM765) in stacking role?

I'd love to hear if anyone has tried it, though!