Public Addresses: '3' vs '1' in front [Bitcoin] I just wanted to confirm, is it true that for a bitcoin (public) address it should always have a '1' in front of it. For certain transactions though I see that there is a '3' in front. How is this possible, and why is it a '3'?
A Bitcoin address prefix is a function of the locking script type for a particular transaction output.

Older Bitcoin Pay-to-Public Key Hash (P2PKH) scripts are associated with addresses that start with the number 1, resulting from a one byte version prefix prior to base58check-encoding being set to 0x00. 

Newer Pay-to-Script Hash (P2SH) transactions start with the number 3, resulting from a one byte version prefix prior to base58check-encoding being set to 0x05.

Examples of P2SH are singe single P2SH signature addresses, multisig addresses, and nested P2SH(P2WPKH) that resulted from segwit to maintain backward compatibility with older wallets that are not explicitly segwit-enabled to wallets that are segwit-enabed. Such addresses conform to the BIP 49 hierarchical deterministic scheme.