Excel VBA - Checking cell value against specific criteria

The first line just highlights what is allowed and I suppose is unnecessary. The idea is to ensure that a) the cell contents is no more or less than three characters b) the cell contents falls in a range from '000' (zeroes) to 'ZZZ' (upper case Z). I previously had two loops, one to check the cell length and another to ensure any text was forced to uppercase. (The contents can be numerial or text, or a mix of). This would have probably achieved the same output but I was informed that the same could be achieved with one loop referring to a 'range' (for lack of a better word) of criteria. I almost got the code to work but it ignored if any of the checked cells was more than three characters. It appeared to check the first three, accept it and move on.
Unfortunately I am checking significantly more than 10 rows otherwise I would not try to seek more efficient code.
Does this explain sufficiently ?

Thanks.