Address 3x is Multisig Address, I think it is difficult to guess this puzzle.
address with prefix 3 is a P2SH address and can be multisig address or segwit address
by looking the txout of this address, I can say this is definitely a segwit address
However, if anybody DOES get the private key for that address, there are still some 72$ (0.01154184 BTC) there.
and based on the info on the site
https://bitcoinchallenge.codes/ posted in the OP,
there is still a puzzle reward of 0.31
BTC unclaimed, but there's no exact pointer of the holding address
however on the other thread, someone mentioned possible related addresses
.. while solving the riddle you can find keys to 3 more BTC addresses with value. They contain 0.1, 0.2 and 0.310 BTC respectively
By looking at associated addresses, this could be:
0.31 - 3NPZiNWiD7cCfXZa1D8tnEZBPgQ884cVw7
0.2 - 1G7qsUy5x9bUd1pRfhVZ7cuB5cMUP4hsfR
0.1 - 1446C8HqMtvWtEgu1JnjwLcPESSruhzkmV