LB-
You can mount all three implements to the X series tractor at once. You do have to use a hydraulic diverter value to "lock" a rear or mid implement in place when not in use. One common oversight for those (including me) who have owned X series tractors is that they can use the I match hitch. JD has a convoluted "A frame" hitch they market for the Xseries machines. It makes mounting rear attachments harder, not easier. I went with I match and to mount a rear implement while using the front was a snap. Your point is well-taken here, though, one reason I "traded up" was the inability to use two attachments at once. I am frequently in situations where I wish to use my box blade and my loader at the same time, which cannot be done with the X series machine. That said, the X 595 I owned would pull the heck out of a box blade and was the stoutest machine for its size I have ever used. For the purpose of your question: yes one can mount multiple attachments at once.
John M