In a 16-oz. jar, combine cream, condensed milk, whole milk, and vanilla. Tightly screw on lid and shake until mixture is thickened and holds its shape for a couple of seconds when drizzled (just before soft peaks form), 3 to 4 minutes.
Add desired mix-ins and mix with a spoon or shake again to distribute. Freeze until solid but still spoonable, about 3 hours (this can be frozen longer, but the best texture is between 3 to 4 hours).
