preform an action base on the 3rd byte, the Msg_In
But, Msg_In is not a byte. Why not?
Serial.println("0x0013");
Why are you sending 8 characters, and calling it a byte? The same question applies to the two other calls, before and after this one (though each sends different numbers of characters).
Your receiving code expects 3 bytes, not the character data you are sending.
Plus what Blue Eyes said.