Adding dimmers to two LED circuits, which have dimmable bulbs - each circuit has three switches
What you have is two three-way switches with a four-way in the middle. What you need to do here, for each switch set, is


Identify which are your three-ways (one black + 2 brass screws), and which is your four-way (four brass screws). Put the four-way back together, as you're not going to mess with it.
Pick which of the three-way switches you want to replace with a dimmer. It doesn't matter which one you pick
Wire your dimmer up to the three-way, making sure to note which wire was connected to the black screw


Your dimmer will work with the existing switches without any other changes but you can generally only have one dimmer per circuit (unless you buy something specialized)