octopi.local is the DNS name provided that the Bonjour service is running. If it isn't running in your local area network and you do have a DNS server running in your local network and your DHCP server has been configured to update the local DNS server and the system you are running the browser on uses that local DNS server then just plain
octopi should work.
Since there are a lot of "ifs" above and you have a monitor connected to the RPi 3, the last line of output after the RPi boots is the IP address it has been assigned. Use that IP address in your browser to connect. If it doesn't print an IP address as the last line, then something is wrong with the (WiFi) network configuration. Note that if you use an Ethernet cable, no RPi side configuration is needed so it should just work and from there, we can help you figure out what is wrong with the WiFi configuration.
Information that would be useful would be the OS of the system you are running the browser on and the name of the browser. Version numbers of these two would be better.
If it is a Windows system, open a command prompt and provide the output of
ping octopi.local, and