Those are passive adapters, the mac pro will only support 2 DVI and/or HDMI displays simultaneously using passive adapters. It is a bit confusing as they look pretty much the same but there is a difference between passive and active.

Here is an amazon link to an active DVI adapter and here is an amazon link to an active HDMI adapter. You should still be able to use 2 passive adapters, the rest will have to be active.