Those are P2SH (Pay-to-script-hash) addresses. You can read more about different address prefixes here: https://en.bitcoin.it/wiki/List_of_address_prefixes
Decimal prefix Hex Example use Leading symbol(s) Example
0 00 Pubkey hash (P2PKH address) 1 17VZNX1SN5NtKa8UQFxwQbFeFc3iqRYhem
5 05 Script hash (P2SH address) 3 3EktnHQD7RiAE6uzMj2ZifT9YgRrkSgzQX
Multisig addresses will be a P2SH address type so they will also start with a 3, because you cannot create an address out of a script unless it is either a P2PKH or P2SH (or segwit script with Bech32 address type). You can send Bitcoin to a multisig script, but if it is not hashed, and put into a pay-to-script-hash script, it cannot be represented as a standard address type.