Hi,
from Zabbix 3.0 Documentation (https://www.zabbix.com/documentation/3.0/manual/installation/install_from_packages/repository_installation) :
# wget http://repo.zabbix.com/zabbix/3.0/debian/pool/main/z/zabbix-release/zabbix-release_3.0-1+stretch_all.deb

but there is no zabbix-release_3.0-1+stretch_all.deb in http://repo.zabbix.com/zabbix/3.0/debian/pool/main/z/zabbix-release/ , for stretch there is only zabbix-release_3.0-2+stretch_all.deb

I would like to suggest to change the package name in manual as the fastest fix option.

Regards,
Aleksejs!