You can try to use the X Virtual Frame Buffer. It will act as a regular X server, but will discard any data that is fed to it. You can install it on Debian with sudo apt-get install xvfb
.
You can use it like this:
/usr/bin/Xvfb :10 & # This will launch Xvfb export DISPLAY=:10.0 /path/to/teamspeak3
You could of course also integrate this in the Teamspeak init script.