can i connect my series 3 apple watch to my iphone 12?

Provided the iphone had ios16.1.1 or later and the Watch is erased first it should pair

Compatibility

From  Wikipedia

watchOS 1 – 3: iPhone 5 or later

watchOS 4 – 5: iPhone 5S or later (GPS Only), iPhone 6 or later (GPS + Cellular)

watchOS 6 – 8: iPhone 6S or later

watchOS 9: iPhone 8 or later

First force restart it

i.e. press and hold both the side button and Digital Crown for at least 10 seconds, then release both buttons when you see the Apple logo.

Then Force Restart the paired iphone   Force restart iPhone – Apple Support (UK)

If you're then able to get into the Watch settings, General, Reset, Erase cintent and settings.

Otherwise Contact Support.

Click here > Contact - Official Apple Support

or phone numbers 

Click here > Contact Apple for support and service - Apple Support

Or visit an Apple Store genius bar