You will have to use a conversion program to S&T files into a different format. GPSU seems to be the most popular. You cannot directly load S&T files into Garmin GPS's. But, if you will download GPSU you can then open the S&T file in GPSU and then save it to the Garmin GDB format. The GDB file can then be opened in MapSource and transferred into your GPS. I'm assuming that the III can accept files transferred from you computer. This works great with my 2610. I hope it's the same for your III.
Hope this helps.