A Strobist I'm guessing?
Hmm, interesting question. The main questions back are:
1) Why do you want one 540 on-camera?
2) Do you care about P-TTL on that 540? I'm guessing not since mixing P-TTL and non-P-TTL flashes simultaneously will likely result in massive wonkiness.
I don't think there's a clean way to do this without a pile of adapters on top of your camera, making it more likely that the flash might accidentally snap something off. (For example, a male-hotshoe-to-PC adapter with a "cold shoe" on top, a PC-cord splitter going to the Pocket Wizard and a female-hotshoe-to-PC adapter with a "cold shoe" on the bottom mated to the previous adapter's "cold shoe".) You might want to consider a flash bracket and a similar pile of adapters - you can get rather sturdy el cheapo ones that screw into your tripod socket for $20-30. Of course, both solutions will make you lose P-TTL.
Also, keep in mind that the AF-540 can be reconfigured to act as a standard optical slave (as opposed to a P-TTL wireless slave). Hold down the Light button for a few seconds and you should see SL1. Hit the S button to change this to SL2. So you would trigger two flashes with the PWs and the third optically from the other two. (Of course if anyone else is taking flash pictures they'll start popping your strobe...)