Nobody seems to have touched upon this... But do you realize that the RX580 card that you get with the Mac Pro doesn't have its own display outputs? If you want to output to three displays, you will be using both of the backside Thunderbolt ports PLUS one of the top ones (now, that's gonna be convinient, a DP-cable going from the top of the machine to a monitor...) And then you have ONE TB output left for everything else. This seems insane to me. Unless it was done to make people buy the more expensive graphics card... AND to encourage you to buy an extra I/O card as well. Wow...
Why couldn't you just daisy-chain the three monitors? They're all Thunderbolt, right?