Can't comment on whether you should get a 3-day or 1-day ticket, however you are in for a great experience. Me and my mate went in 2006 (drove and camped). Awesome few days, if you get chance try and get onto the old track, it's an experience (but mostly fenced off).
We sat at Variante Ascari which was great, but I'd probably go for the first corner if I was going again.
Oh yes, and pack sun cream, it's really hot!!