With radio triggers, there is no need to have a slave mode to get them to fire because they will fire through the attached receiver. The way they work is that you have a transmitter attached to the hot shoe of your camera--this sends the signal to each of the flash units you have set up in your shot. Then on each flash unit you want to fire, you will have a receiver attached -- the receiver receives the signal from the transmitter which in turns fires the flash. Simple.
Some flash units have a built in optical slave, for example, the Yongnuo YN 560 I and the YN 560 II units. These are terrific because you can set them to slave mode and they will fire when they see the other flashes fire, and you don't necessarily need a receiver attached to each of them. For example, let's say you own just 1 transmitter and 1 receiver, but you have 3 YN 560 II units and you want to use all of them in a single shot. You can attach the transmitter to the camera, and your receiver to one of the flash units. Then on the other 2 flash units, you can set them both on slave and they will fire when the first unit fires, but they will not need the receiver because they are firing courtesy of their optical slave. Simple.
Note the Pocket Wizard below is a "transceiver" which works both as a transmitter or as a receiver, which makes it super flexible. Not everything is a transceiver though.