diffusing flash ? you can use the cheapest method.. cover your flash with tissue paper..
as for the slave flash issue, although there is no way to mount on top of your pop up flash..
the Canon HF-DC1 "high powered flash" that is supposed to be a flash accessory for your S3 IS, it has an included bracket that can be attached to your tripod mount..
ah ok. How do I go about sticking tissue paper on the flash? how many layers do I need? Sorry for asking some dumb questions ;p