If there were 2 dummies....it wasnt the umpires...
except maybe for the umpire who said this was a new rule......this is not new.....
A pitcher may throw to an unoccupied base in order to make a play..........that is not a balk....
Often the problem comes in when people read the rule book incompletely....the rule says it is a balk to throw to an unoccupied base....(then adds) except to make a play........some only read the first part and not the second....if the rule was written correctly it would state, Except to make a play, it is a balk to throw to an unoccupied base......
Corresponding NFHS rule...6-2 Article 4 B.
Balk if there is a runner or runners, any of the following acts by a pitcher while he is touching the pitchers's plate is a balk.
B. Failing to step with the non-pivot foot directly toward a base (occupied or unoccupied) when throwing or feinting there in an attempt to put out, or drive back a runner: or throwing or feinting to any unoccupied base when it is not an attempt to put out or drive back a runner...
He stepped directly toward 3rd....and it was an attempt to make a play on a runner (you)....
Unfortunately you are out and your coach was wrong...
PS...as a baserunner never stop running until an umpire tells you its a balk.....