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Directions for the dimmer say that you can use only one dimmer per circuit. What defines a circuit? Does each breaker control one circuit? Or is a circuit one complete element - one switch and a light fixture(s)? Specifically, I have installed a dimmer which controls a light. It is not a 3-way because there is only one switch. Controlled by the same breaker are several other switches, including a 3-way that I would like to install a dimmer on? Can I?
In this case, circuit refers to a group of lights controlled by one or more switches. I prefer the term zone. Your living room is probably fed by a single breaker. You may have 3 zones of light (chandelier, recess, sconces) on that circuit but controlled by 3 different switches. You can have a dimmer on each one of those zones.
If the chandelier is controlled from 2 locations, both switches are 3-ways. If it is controlled from 3 or more locations, there are 4-way switches between (as the wire goes) the 3-ways. In either case, only 1 of the switches can be a dimmer. Mechanical dimmers like Diva and Ariadni will work with existing mechanical 3-way and 4-way switches. However, the dimmer must replace one of the 3-way switches. You can only dim from the dimmer location and have on/off control from the other locations.
Some electronic switches, MACL-153M for example, will work with a single, mechanical, 3-way switch. If you have more than 2 locations you use a companion dimmer (MA-R). The advantage of the companion is that you can dim from all locations. You can also use a companion dimmer if you have just 2 locations.