How to set up a runlevel 3 in Debian 8?

Two things you need to know:

1) Systemd boots towards the target given by "default.target". This is typically a symbolic link to the actual target file.

2) Systemd keeps it's targets in /lib/systemd/system and /etc/systemd/system. A file in /etc/systemd/system takes precedence over those shipped with the OS in /lib/systemd/system -- the intent is that /etc/systemd is used by systems administrators and /lib/systemd is used by distributions.

Debian as-shipped boots towards the graphical target. You can see this yourself:

$ ls -l /etc/systemd/system/default.target
... No such file or directory
$ ls -l /lib/systemd/system/default.target
... /lib/systemd/system/default.target -> graphical.target

So to boot towards the multiuser target all you need do is to put in own target:

$ cd /etc/systemd/system/
$ sudo ln -s /lib/systemd/system/multi-user.target default.target

It is highly recommended not to mess with the manual symlink-ing, but rather use appropriate options of the systemctl command. In this case, to set the default target you should run:

# systemctl set-default multi-user.target