I was kinda thinking about taking Calc III during the summer as well, but I think I'm going to pass. For me, I can pass a math course that is rushed through, but the light bulbs just don't go off until the information sinks in... basically what I mean is, a lot of the stuff I learn in summer courses I soon forget. But I'm sure if you're willing to work hard enough you can do it, also if the professors are better than average, then go for it. I'm not sure which is worse, forgetting some of the information or getting stuck with a bad professor & just not learning half of it (unless you're really good at self study).