Not a Drop 3rd Strike...

[RKBUmp]

The closest I can give you is NFHS case play 3-6-13 Sit B. It involves a batter advancing on ball 3 and the runner also advancing, but same basic principle. The defense is responsible for knowing the count and making the appropriate play. ASA has basically the same clarification that the defense is responsible for knowing the count.

As to your exact play, I had this situation happen in a game I was helping my daughter coach last year and the umpire called the batter out. I argued my case to no avail and eventualy contacted the state rules interpreter citing the case play referenced. He confirmed this would be the same type of situation and the batter should not have been called for interference as the defense should have known the count and played on the runner advancing from 3rd.