evanrich:
good news! Tested your code, and it works for 3 displays. I can't really tell a speed increase or not, but with 3 panels it scrolls across all 3 in the correct order, and the issue with the dots trailing the text when scrolling right is gone as well! Congrats!
This is a really good news for me XD I cannot get rid of the issue with 3 panels. It doesn't seems to be an hardware problem: the same code perfectly works with 3 display in ChipKIT Uno32! 
Nobody here lent me an oscilloscope... the thing I should try is to add pullup resistors at least on CS and CLK (the issues w/ > 2 panels is related to chip selection thru 74164, not the ht1632c themselves).