Do I need to charge the watch first?

Back to your original question, if you are pairing a series 3 with an iphone 8 or newer on ios16, it is not straightforward, so make sure of the following

• Ensure the phone is on iOS16.3 or later
• Set it up as a New Watch, do not attempt to restore any backup
• If necessary erase the Watch and start again

Simple erase if you need it

Put your watch on its charger and keep it there until you complete these steps. (select the watch OS version 8)

Erase Apple Watch – Apple Support (UK)

I would, yes

You're welcome, glad I could help.