systemd has the concept of targets as a more flexible replacement for runlevels in sysvinit.
Runlevel 3 is emulated by multi-user.target. runlevel3.target is a symbolic link to multi-user.target.
You can switch to 'runlevel 3' by running
# systemctl isolate multi-user.target
Another option is to use kernel command line arguments.
On boot, systemd activates the target unit default.target whose job is to activate services and other units by pulling them in via dependencies. systemd parses its own kernel command line argument
This may be used to temporarily boot into the multi-user.target boot unit: