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.

Found what I wanted. Solution was an Active mini DP to HDMI adapter. I was under the assumption the first adapter I bought was active, but it wasn’t. This is what I used and it works great. Full 3440x1440 over my monitors HDMI port (the monitor’s port limits it 100Hz over HDMI, but that’s fine for my use case with this machine)