I was in a similar position and went with the st-e3 and 600ex to start. Then added 3 more 600ex. I use the Foursquare setup and a parabolic outdoors and it does a great job in daylight. The High Speed Sync built into this setup works very well and is what allows for beautiful fill during bright daylight. This setup is also flexible in that you can take 1+ flashes off the Foursquare and have a multiple light setup with Manfrotto clamps or Gorillapods (both lightweight). With your setup (5d3 and 600ex) you won't need Pocket Wizards unless you start using portable strobes like Elinchrom Ranger RX and need the HSS.