Do all input addresses in a transaction always belong to same user?

Yep, they do belong the the same user(wallet) as far as I know.

Code:

18KUnn8L8DhiASGNNJbC5V49MBQTy6uFVu
1BZZEdvuP5qKPLq7wbXcZ6f3zBMbhqzJRg
12XpnNjijxV5sa1xiaD7XE9QudcWKLQhbZ

Yep. However, you can also verify them here-> https://www.walletexplorer.com/wallet/11e5dada77ed81bc/addresses . You can therefore say that the three addresses(given above) are a part of the same wallet [11e5dada77].

Thanks, are you aware of any other service which also gives the wallet id for an address preferably with api.

Thanks, exactly what I was afraid of.