With some radio slaves it might be possible to wire another flash to the receiver, however it is a much better idea to have a receiver for each flash. I would also recommend that you use receivers on all flashes and not try having 2 wireless and one wired. The Nikon D5100 does not have a PC plug to connect external flashes, so you would have to have a hot shoe adapter (or flash cable such as the Nikon SC-29) to allow connecting an external flash, then put the wireless transmitter in the adaptor.
As far as what you can do with a particular radio slave kit depends greatly on which wireless system and what flashes you choose. At the basic level the wireless system fires the flashes when you press the shutter. I have had good luck with Nikon SB flashes and PocketWizard Flex-TT5 system. That system allows you to set the power of the remote flashes and/or use iTTL exposure with the wireless system. That ability has been very helpful in speeding up the photo process. The amount of time required to goto and set the power on a flash quickly adds up.
I have a photograph of some if the equipment I used to rephotograph the first underground photograph in America.