I've seen on videos lately dudes using on camera flash as well as external.
I'd personally recommend just not going on camera in any situation unless there's literally nothing else you can do.
I used two flashes the whole time i shot flashed skate photos, it's easier than three and can be more effective.
I'm at work right now so can't link the photo but i posted on here, about 3/4 of the way down, how to shoot with two flashes. It's all about that 180 degree line.
that's a good thread to learn from too regarding zoom levels and stuff. I don't want to be mean to Tin Man but his photos aren't good to learn from. Look at skate videos and pause them when you see flashguns to see exactly how the pros do it. There's alot of people in skateboard photography who think they know how to use flashes but they don't, so be careful who you choose to learn from.