Thanks for posting! I am sorry to know that you are having trouble getting the third monitor to display when connected to the Plugable UGA-125.
Connecting multiple monitors through multiple units of our USB 2.0 Graphics Adapters is very common. So it is definitely possible to use 2 adapters to add 2 monitors.
Could you give us some more information regarding your setup so that we understand why you are unable to connect the the third monitor using the 2nd Plugable UGA-125?
Are the Plugable UGA-125 connected directly to the inbuilt USB ports of the system or are they connected through a Hub?
Is the OS reporting any errors when you plug in the 2nd Plugable UGA-125?
Does the Green LED get lit on both the units of the Plugable UGA-125?
Once you post back with this information, we will figure out the next steps.