I need to assemble a large RAID 6 volume, with 24x 4TB, considering 2 of them as hot spares. It's being very hard to find expanders/backplanes here around, but I can afford to have three RS2BL080. Would they work together to allow a single large VD?
The RS2BL080 supports up to 32 physical devices, and you can use it together with the expander http://ark.intel.com/products/64080/Intel-RAID-Expander-RES2CV360 RES2CV360. But you can't create a single large VD across three RAID controllers.
You may also want to take a look at the http://www.intel.com/p/en_US/support/highlights/server/rc-rs2sg244 Intel® RAID Controller RS2SG244 which has 24 internal ports.