The relay module is often used for high voltage, but really it's for anything at a specific voltage not supplied by the Tessel– it just routes the power through a switch. So if you need exactly 3.0V, the best thing would be to route your battery power through a relay module to make Tessel switch it on or off.
That said, the Tessel operates natively at 3.3V, which is not far off of the 3.0V you need. Depending on the tolerance of your air freshener, you might be able to get away with just toggling a digital pin, like I did in this project. You would plug the ground line into any of the GND pins, and switch whichever digital pin you choose on and off.
You can find a table of all of the pins here.
Let me know if that needs more clarification!