RE: 2.4G systems
I went over to the HK site andf found out that, yes the 3 channel rx is only compatible with their GT-2 3 channel surface transmitter. Also since the 4 channel transmitter is not a multi-model programmable TX you may need to bind the rx to the tx each time you fly a different model. In case you're unfamiliar with this process, binding the RX to the TX insures that the RX only takes commands from your tx. It appears from what you said about the TX having a range/bind button is it works like my Spektrum system. The bind plug is put into the RX, usually in the battery plug position, and the receiver is powered up. The bind switch on the TX is pushed in and held while the TX power switch is turned on. The bind button is held until the rx binds to the TX. On my Spektrum system the light on the rx will flash until it binds at which time it stays on steady. The TX and RX are then turned off and the bind plug is removed at which time the system is ready to go. If you have more than one RX you may need to do this every time you switch models. On a multi-model system it isn't necessary as switching the TX to the model you're now flying automatically resets everything to the model in question. Hope this helps.
Mike