Why is Craft CMS attempting to connect with SSL3
The error is coming from when Craft tries to send out an activation email for the new user.

In Craft's email settings (Settings->Email), you're telling Craft to connect to your email provider over SSL (TLS), but it looks like your local OpenSSL certificate config is setup improperly on your box.

More info here: https://github.com/PHPMailer/PHPMailer/wiki/Troubleshooting#php-56-certificate-verification-failure

I definitely wouldn't recommend their workaround of disabling SSL certificate verification as that opens up a slew of security issues, but would fix the local OpenSSL configuration.