This tutorial is a nice example of using speedlights outdoors, though not the best example of a tutorial. It would have been great if they had included info about the shutter speed and ISO, things integral to creating the shot. But you can make a few guesses: it's an overcast day, so using the Sunny 16 Rule the optimal exposure was probably around F/8 (given matching shutter speed and ISO, 200/200). In order to freeze the hair in the wind the shutter is at least 100. Typically, the SB900 at close quarters can equal the sun but not over-power it for a portraiture. Add a second SB900 and you've doubled the light-- or raised the F stop by 1. So if a single SB900 can output the equivalent of F/16 at close range, than two will be F/11, and three SB900 will be about f/9. But Joe used TTL, so we have no idea whether the strobes were firing at full, half, or quarter power. We only know he had enough light. The natural light exists at about F/8; he shot at F/16; and a single strobe would light the face at f/16. So he didn't really need the other two-- except that the diffusion panel reduced the light and the distance from subject to strobe appears to be about 6 feet. In all likelihood, he needed the second strobe, and the third was icing on the cake... helping his SB900s recycle quicker. But these are just arm-chair estimates. It would be nice to have learned exactly what he did.

The video displayed the F stop and ISO information at 00:29. He used f/13 at ISO 200 at 1/200th sec shutter speed.