Firstly, the III+ is NOT for car navigation. While you can get MapSource data for streets, it doesn't have a road routing function and thus won't work well for car navigation. It was originally designed for use in boats, but also works well for backcountry navigation. While there are now smaller lighter units, most of which have color screens, the 3+ still works well. It also has limited memory which is its biggest problem.
It was updated with the V (5), but it is almost identical save the way you load maps (you load fairly sizeable chunks of the country, but must pay to change what region is loaded.) The III+ on the other hand has far less onboard memory, usually only enough to load a 20mi x 20mi area of topo, but once you buy the TOPO MapSource data, you can load whatever you want as many times as you want.
The biggest (perhaps only) advantage over the newer smaller units, especially those with a color screen is the ability to use an external antenna. This is especially useful when driving, but provides better reception when hiking too (you can simply put the antenna in your pack).