3 way dimmer switch - need help
Neutral
Ordinary switches don't need neutral. Many (but not all) dimmers, timers, smart switches, motion detectors, etc. need neutral. There are some exceptions that:

Leak a little current through the circuit when &quot;off&quot; - works fine with incandescent but often not very well with LEDs
Use ground - permitted for very low current in limited circumstances. But you can't just &quot;use ground instead of neutral&quot;
Battery backup

but a lot of devices now need neutral and hot in the box, so that they can complete a circuit to power the switch itself even when the lights are off. Because of that, code now requires neutral to be available (or easily added via conduit, but the cables you described are not a conduit installation). But older houses often don't have neutral in switch boxes.
Based on your description, you have either 2 travelers + hot (i.e., first switch in the group, so hot comes in from the panel and travelers go out to the 2nd switch) or 2 travelers + switched hot (i.e., last switch in the group, so travelers come in from another switch and switched hot goes out to the fixture). But in any case, it doesn't sound like you have neutral available.
Take a look at the other 3-way switch. You may have better luck there - i.e., 2 cables - one to this switch and another either to the panel or to the fixture. There are a few different possibilities, including:

Panel -&gt; Switch 1 -&gt; Switch 2 -&gt; Fixture

and

Panel -&gt; Fixture -&gt; Switch 1 -&gt; Switch 2

and depending on a number of factors, you may or may not have neutral available in the other switch box. If you do, then install the dimmer there. If you don't, then you have to do some rewiring to make this work. You can't just add a neutral wire - you have to replace (e.g., 2 wire + ground with 3 wire + ground, or 3 wire + ground with 4 wire + ground) entire cables.