How to unlock my Apple Watch 3 series that I bought from someone

If it is asking for the Apple ID password from a previous user, you will need that user to either give you their password or remove the watch from their Apple ID

What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support

The person that you bought/got this watch from needs to follow all the steps in this link;

Remove the Activation Lock so that the buyer can use it.

If they can't or won't then get your money back or recycle the watch and mark it down as a bad experience.