http://iscoresports.com/

Here's the situation:t9 no outs bases empty. Grounder down the 3rd base line, backhanded and booted by 3b, he then hurries the throw which goes into the stands. Single and throwing error or fielding and throwing error?

It is just one error by the third baseman. You have to make the judgment call if it was a single and an error or just reached on error, but it is only one error in either case.

To echo support, That is how official scoreing rules are defined. Only one error.

If you are following different guidlines, the software lets you add a error anytime under misc menu.