New member here! Loving the platform. Would be great to have 3rd touchscreen support on C3500 and secondary touch support for C1500/2500. 
You can use a touch screen already. It must be a model with the same resolution as the internal one.
This is true however 3rd monitor touch isn’t supported. If you use an external monitor with touch feature the calibration is off as the touch is mirrored to the second screen (C3500). I don’t know what happens on a single screen but I assume it’s the same issue.