I wouldn't do a one time trip in that short of an amount of time, but for a first trip, I'd say three days will give you a good sample of what WDW is like.
If you've only got three days or less...you're going to miss out. If you go for six days, you're going to have to skip some things - that's actually one of thethings i love about WDW. There's always something more to do there, although some DISers stayfor ten days or longer, but that's just not our style.
With three days, I'd choose a day at MK, a day at Epcot, and a day split between AK and MGM. If you don't want to pay extra for the hopping, then I'd pick one of the last two parks, on the day that it has the longest hours. I'd want to take full advantage of EMH hours, so I'd stay on property, preferably during the week, and perferable NOT during a popular holiday week -such as Christmas, President's day week (Feb.) , or Easter. I'd also probably start plotting my next trip as soon as I got back.
As long as I wasn't traveling with little kids, I'd use fastpasses, and try to goon as many of the fastpass attractions as I could, as well as those that looked interesting to me. With little kids, I'd do all the attractions in the preschool section of the Disney web page, and then have the adults take turns going on the adult attractions using fastpasses.
I'd avoid using the
Disney Dining plan, and opt for mostly quick service eateries, I'd pick a resort with a food court, but spend as much times in the parks as I could. Eat aquick breakfast at the food court, then get to a park ASAP via the resort's main bus stop. Eitehr that or eat breakfast on the go in hte parks, the same with lunch. I'm often big on having a sit down dinner, but with asort amount of time, I limit myself to one nice table service meal. I'd also limit the amount of time I wasted over the day - when one person uses the restroom, everyone attempts to go - or you make a trp while others are finishing their lunch. I'd only buy snacks at booths when there were nolines, and I'd eat my lunch at 11:30 instead of noon.
You can find out the park hours on the Disney webpage, as well as get an idea of many of the attractions, and the many dining options.