The reason idents for radio navaids (VOR/NDB) are only three characters is because they are broadcast via morse code. They need to be copyable by pilots who are

The reason idents for radio navaids (VOR/NDB) are only three characters is because they are broadcast via morse code. They need to be copyable by pilots who are otherwise somewhat busy and not particularly proficient in Morse. For this purpose, they only need to be unique to that frequency within plausible radio range. 'nm' and 'NM' are the accepted abbreviations for nautical miles in the aviation industry, whether official or not.