Okay, I don't know if this will work, because I couldn't make it work on mine because of the language version issue. But, if you add the format to the formula, just like I showed you before, and then you ALSO put square brackets around the fields like:
Format([Field1]+[Field2]+[Field3],"0,00")
That might be why it's not working. But, since I don't have a copy of Access in your language, mine doesn't like the characters that are currently in yours. So, hopefully this will help. Sorry I can't be more definitive.