Adding rpcport=18332 temporarily solve your problem. If you want to run mainnet or testnet at the same time when regtest is running, you wont be able to use them, it is because when you assign
rpcport=xyz all bitcoin instances will try to interact through same rpcport when daemon is started but at one time only single daemon can access single port number.But in your case all three daemons tries to take rpcport as 18332.
Infact what you can do is, try to get connected to default rpc port for regtest by typing command:
bitcoind -regtest -daemon -rpcport=18443
Note:For bitcore v15 rpc port is same for testnet and mainnet where as in v16 rpc port is diffrent for all of them