Remember also that a softball is bigger than a baseball. This may be another reason that you are perceiving the the softball strike zone is so much bigger.
As far as three-umpire mechanics, the NFHS books are a good source. Here is another, online, source that you can look at as well:
http://www.cactusumpires.com/pages/sub/3ump.html
Good luck with your softball umpiring and stop by here often!