Coach here. I hate poetry but I do like poking umpires with a stick! Below is the best site for your question.
http://eteamz.active.com/fastpitch/boards/fastpitch/
Three ways to cover third ranked in my order of preference.
1. If pitcher or third baseman fields the ball, catcher covers third. Third baseman takes home.
2. Catcher stays home, pitcher or third baseman (whoever does not field the ball covers third.)
3. Left field covers third.
I like the first option because I want to keep my pitcher out of harms way. If I finally find one who's mother doesn't wear me out and who's dad doesn't constantly rag on the umps, I don't want to lose her to a broken finger on a tag.
Option 3 is too dangerous. We never throw a ball to a bag that does not have outfield backup.
Next time your umps blow a call or a rules interpretation, bring it to these guys on the board and you will find that 99% of the time it is you that was mistaken.