On the intial setup, did each transmitter report to the display correctly, as channel1, channel2, channel3? Did this also setup the same on the other 2 displays?
Does this problem occur on all 3 displays? Perhaps shutting down 2 of them and just run 1 display to see if it's an interference problem from the other displays running at the same time. If it happens on only 1 of the displays possibly a sticky 'CH/+' button?
According to the manual, if the signal is lost you should be able to press and hold the 'CH/+' button while on it's channel to reacquire the signal.