Yep, you can. There are manual modes on the S3 - you'll want to use "AV" and then you can set the aperature to whatever you want and it'll adjust the shutter speed.
I got my S3 about a month ago, and I'm still learning. So far I've used it a 3 band competitions; I use the "sports" setting - it's supposed to give a faster shutter speed to stop action, and it also has something I LOVE - continuous shooting. If you hold the shutter button down, it'll take a picture every 1.5 seconds or so (the manual says you can adjust it, but I haven't tried that yet). So you don't get that delay that you usually get with digital cameras, where it'll display the picture you just took before you can take another.
Here are some pix from the first competition where I used my S3:
The band lined up and ready to go:
My DS14 and his bari sax:
My DS16 during his trumpet solo:
It was pouring down rain that day, so the pictures aren't as crisp as I'd like, but you can certainly see the power of that 12x zoom!
I've noticed during the night-time competitions that the pictures get grainier as you increase the zoom, so I tend to like the full-field pix a little better. But for proud parents, who cares about grainy pix when you're seeing your kid?!