The Slumber Party Massacre series will never be known as one of the most life changing horror franchises of all time. They're mindless fun at best and Slumber Party Massacre III attempts to do something slightly different by keeping the gratuitous boob shots and gore, but taking away a lot of the fun.
The basic set up is typical slasher stuff with a group of girls hanging out on a beach and getting ready for a big sleepover at one of their parents' houses that night. They're semi-stalked by a creepy guy and, on the way back to the house, one of the friends is murdered. Obviously, once the party kicks off, a few boys show up to crash the party and check out the girls dancing in their underwear (and less) and the killer decides to crash it, too.
Unlike the first two films in the series, there's an element of whodunit mystery here. Could the killer be the aforementioned creepy beach guy with the ponytail? Perhaps the nosy neighbor across the street who spies on the girls with a telescope and seems really interested in checking out the house to see if he wants to buy it? Or maybe it's one of the goofy guys who have showed up to crash the party? This element does help it stick out from the other two films and keeps things pretty fun at first.
It's only once the killer is revealed that things take a slightly darker turn and it begins to feel more home invasion than slasher. If you're into that sort of thing, maybe you'll enjoy it, but I found it left a slightly bitter taste in my mouth.