AFAIK...the N73 is not a 3rd edition symbian phone, it's 2nd, the same as the N70. The N80 is 3rd edition. I think I read somewhere that TomTom Mobile 6 has recently been released and will work on 3rd edition symbian phones but I don't know for sure. TomTom Mobile 5 will also work on 2nd Edition.
Hope this helps.