The first one is the only one that works. The others don't make sense.

Standard business attire that is inappropriate for even short visits to the production area, we are sorry for the inconvenience, "The Company" <insert name> This is how it would be written in American English.

In all of the sentences, "determined" is a poor word choice because it is open to different interpretations. Better choices are "decided that attire is...", "chosen attire that is...", "selected attire which is...", depending upon what you mean to say. You must decide what the company has determined. Has it determined a class of clothing, or has it determined that something is inappropriate. In the end, the two choices might mean the same thing, but they lead to different grammatical constructions. Only #1 is really good.

1/3 - determined standard business attire ( in general ). 2 - determined standard business attire ( specific ).

You are right- difference is very small. #1 = all standard business attire is inappropriate. #2 and #3 = if the average employee read this, they would be confused and wonder, does this mean only certain kinds of standard business attire are inappropriate? If yes, then what kind? What can I wear that's ok and what's not ok?