I just got off the phone after an hour discussion with AppleCare. I talked to two levels of support techs & they said two Mini DisplayPort to Dual-Link DVI adapters are indeed required to get three DVI monitors to work - at $99 each.
I also tried connecting a single DVI-to-VGA adapter with two Mini DisplayPort to DVI adapters and that still only gave me two working monitors.
It still seems odd that it requires TWO of the $99 adapters to work. If you look at ATI's 5870 specs (
http://www.amd.com/us/products/desktop/graphics/ati-radeon-hd-5000/hd-5870/Pages /ati-radeon-hd-5870-specifications.aspx ), it looks like the standard 5870 config only requires one DisplayPort display if you want to use three displays. I asked AppleCare about this difference between their cards & the ATI spec & they didn't have an answer why there was a difference. Maybe I'm misinterpreting the spec.
I asked them to put in a request to change their Mac Pro Tech Specs page to reflect the additional parts required to get three monitors working correctly. If I had known that I needed to buy $200 worth of connectors, I would have bought two 5770s instead of one 5870.
On a related note, I asked about adding another graphics card to my Mac Pro. AppleCare told me that because the 5870 uses two power adapters, you can't add another 5770 or 5870 to the Pro. They said a 2010 Mac Pro with a 5870 "is maxed out."
I may try to find a SATA power plug to PCIe power adapter and drop in an 8800GT I have. I'm not sure if it'll work though - the 5870 takes up a lot of room & there might not be room to get another card in the other 16x PCIe slot.
A friend suggested I try MonoPrice's Mini DisplayPort to Dual-Link DVI adapters. They're $65 each:
http://www.monoprice.com/products/product.asp?cid=104&cp_id=10428&cs_id=1042802&pid=6904&seq=1&format=2 If the 8800GT doesn't work, I'll try to the MonoPrice adapters.
For now, I guess I'll be happy with two of my three monitors working.
-Bryan