May I suggest and I don't know if you can do it this way but changing the local var to Text and in the expression you can use:
FormatDecimal(TextToDecimal(Replace(Decimal3, ",", ".")), 2, ",", ".")
The FormatDecimal is to display to the user with the comma again, the TextToDecimal can be used to send the value to the server or you can do it server side. It seams having the comma is the problem.