Apple Watch 3

You probably need the following settings to be On/Activated

Passcode (must be activated)

Wrist Detection (Settings / Passcode)

Unlock with iPhone (Settings / Passcode)

This should enable you to unlock once only after putting the watch on your wrist, and it will also unlock by next time you unlock your phone if more convenient so you don’t need to tap the passcode into the screen.

If your watch persists in locking while you are wearing it check the band is correct and sufficiently snug so it continues to see your arm, Wearing your Apple Watch - Apple Support. If this doesn’t help there is a known problem if you have tattoos under the watch which mess with the light sensors used by wrist detection - but if this is not relevant also consider your watch may simply need cleaning, or it could have a hardware problem which needs checking by Apple Service.

Pass your disappointment direct to Apple, because there is almost zero mention of this known problem on the watch pages of the website. They could maybe make it a little more visible for potential buyers. Feedback page is at https://www.apple.com/feedback/watch.html

Meanwhile... the only workaround seems to rely on your other arm not having a tattoo in the critical area. I know, not ideal...