The 15.3(3)JPN train is widely regarded in the networking community as one of the most stable releases for autonomous/conversion use cases for the 3700 series.

To fix your TFTP/boot attempt:

It is highly unusual to encounter a string like ap3g2k9w7tar1533jpn1tar better in a natural language context. At first glance, this appears to be a concatenation of product codes, hardware identifiers, or cryptographic hash fragments.