How to login ubuntu core 18 raspberry pi 3 in headless startup

I have ubuntu core 18 image on Raspberry Pi 3 B+. I don't have monitor. Can someone help me to login to ubuntu core via SSH. What is the default ip address, username password?

I'm afraid that the generic Ubuntu Core images requires a monitor and keyboard to enable SSH access (see the directions). Ubuntu Core's philosophy is "secure by default", which means there IS no default password. You must create an Ubuntu SSO account and upload your public SSH key, and then use the keyboard and monitor to provide the email address for your SSO account and it'll fetch your public key. Once that is complete, you no longer need the keyboard and monitor and you can SSH in.

you can install unbuntu desktop headless on pi 4 using TTL to USB device, so on your laptop and pi will be connected using the TTL device, no monitor or keyboard needed:

https://limesdr.ru/en/2020/10/17/rpi4-headless-ubuntu/