None of them measure calcium. With the base AC2 you get pH, Temp, and ORP monitoring, plus control of your pumps, lights, etc. using X10 modules. The AC2 pro adds dissolved oxygen monitoring (interesting, but not particularly useful) and conductivity, which is used to estimate salinity (but not accurately enough to use for top off).
The AC3 does have a few more features, mainly the web stuff I mentioned, but no more measurement capabilities. It's a cool toy for sure, but if you want to get the 80% of the functionality that most of us use for half the price, go with the AC2 (be sure to get the backlit display for $30 extra). For $500-600 total, you can have a setup that does everything you need.
You can get different packages - I got the one with just the pH probe, because I don't need to monitor ORP (really only useful if you are using ozone). Most of the packages also come with the basic X10 control stuff you need, and a couple of modules. You can get extra control modules from Radio Shack or online for $12-15 each.
To hook it up to your computer (completely optional, but very cool), you need the cable (~$15) and most likely the ground isolator ($40). Try it without the isolator first - if you're lucky you won't have a problem and can save the $40. You also need software - you can either use the Aquanotes software from Neptune, which is $150, or you can use JAqua, which does 95% the same stuff and is shareware - registration is $30 I believe. You also need a computer of course - I didn't want to expose our main computer to the Internet, so I got an old laptop from work that just runs this software.
Next time you're in town, you're welcome to come by and check mine out if you want to see one in person before spending the bucks.