In the same node model, you can have many radio transmitter receiver objects. You can start from an LTE node model, and you can add "wireless_lan_mac" process model from the Wi-Fi node model and the related radio transmitter and receiver objects. Then you add the WiMAX process model. You will need an additional process model to manage all the three access technologies, it will serve as an interface with higher layers (buffering, deciding which technology to use to transmit or to receive ...).

Please take a look at my publication : https://www.researchgate.net/publication/269269218_A_generic_mobile_node_architecture_for_multi-interface_heterogenous_wireless_link_layer

I have implement this on OMNeT++.

Wish this will help you.

Conference Paper A generic mobile node architecture for multi-interface heter...

Thanks Rafaa. However , I am still looking for a help to do this in OPNET!