Multiple Monitor Use, I am looking for a 3 monitor setup

There is only one "con" to multiple monitor setups.  That is, you will have chronic neck pain and eye strain if an ergonomic layout is not used. There are plenty of articles about this on the web including recommendations by OSHA.
I am sure that you have already had occasion to think when another monitor would have come in handy.  However, by asking about the pros and cons, it seems that you are asking about the value of the third monitor, the bang for the buck.
Once you have chosen an ergonomic layout, you can determine the cost of equipment and changing your work space. Cardboard mockups and tape outlines always helped me visualize space requirements. Now ask the piercing question "Will the third monitor help me make more money with my music?" or even "Can I use the money for a third monitor setup to buy another piece of audio equipment instead?"  Answer honestly and you will be happy with your choice.  

I went from 2 1920x1080s to a large 4K, which is like 4 1920s 2 x 2 without gaps/frames.  One of my original 1920s is off to the side for reference docs and such.
 
 

In a somewhat related note. I have three monitors at work (not a DAW). One of the three is on a SVGA->USB adapter plugged into a USB port. Another is on the DVI port of the Vid card and the last one is a an SVGA->HDMI adapter that go into the same Vid card as the DVI. To be clear the DVI and HMDI ports are on a vid card not vid ports of the MB.
 
One problem I have is at least several times a week the 3rd monitor (USB) goes black. If I unplug the USB cable and plug it back in the 3rd monitor comes right back online.
 
This happens no matter what USB (2.0) port it's plugged in to or even if I swap physical monitors.