A strategy question

I noticed last night that one of the many long runs by B.C. last night came when the Als had a three-man rush.
I got to thinking that it would be simple enough to call an audible at the line when they see a threee-man rush. Run the ball. There are only 3 d linemen and the other nine have dropped off into pass coverage. There would be a lot of open real estate .
I can see that a lot of contributors to this forum have an extensive knowledge of the complexities of this game. It seems obvious that offences should have a run option like this, yet I haven't really seen it put into practice.
Why don't you think this would work?