Don't try to log in to your Google account on that device. Log in to Google with what you are using now or another pc etc. Get a prompt for a new password.
Look in Device Manager on your Google account, if you had used this device before, and see if it is listed.
However, I don't think changing your Google password will help unlock your Pixel 3 at this stage. That is something I'm not knowledgeable on.
Normally it takes 24 to 48 hours for a password change to become active, although I may be confusing this with something else and talking through my hat.
Here's the crux (sorry if English is not your first language) :
Can you tell how you have this phone now?
That is, have you used it in the past and forgot about it - but now want to activate it for yourself again or for someone else.
Did you recently buy it online or elsewhere and now doubt the seller?
Did you find this device?
It will help to know - although as mentioned at this stage you are probably out of luck.
I am curious if you can log in to the Google account you had on the Pixel on a different device.