Are addresses which start with 3 multisig addresses? I've just downloaded bitcoin-core 0.17.1 and it seems all the new receiving addresses I create start with 3 instead of 1.

I always thought addresses which start with 3 were multisig addresses.  Anyone know why default non multisig addresses are now 3 and what does this mean?
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.