Update Apple Watch series 3

Apple Watch Series 3 tends to be memory-constrained, so Apple suggests that folks encountering issues with updates use unpairing and re-pairing to update.

Click or tap on the following blue-text link for details > If you don’t have enough space to update your Apple Watch - Apple Support

Here is the more typical procedure > Update your Apple Watch - Apple Support

No a watch cannot be updated unless it's paired.