you'll about 175cm of desk space. You may be able to do with less if you get a triple monitor mount and let your monitors hang over the edge. If you plan on putting the r4 on top of the desk you'll need about 200cm of desk. You should be able to find the monitors dimensions online. just multiply by 3 and add the width of the case if your going to put it on top.
For reference my 3 x 23 inch dell ultra sharp are mounted with a very slight curve. They are 170cm across.