Can a dimmer switch be used in this circuit?
Achievement Unlocked: Wire a box with 6 different wire functions without using any phase tape

Sorry, no points for that &quot;achievement&quot;, just a lot of brain pain.  Let's try this.

Ah, suddenly I can hear myself think
Most of this looks pretty straight forward.  The top /3 cable is the supply side of a 3-way circuit.  The bottom /3 cable supplies light and fan in the usual manner.
But something rather squirreley is going on with cables 2 and 3 here.   You've labeled cable #3 as &quot;Line&quot; but then, feeding all the switches off the black wire of cable 2 doesn't make any sense.
So this raises more questions than answers.
What might make more sense is if cable 2 was actually line, and comprised a multi-wire branch circuit, with one pole serving all loads here, and the other pole heading off to parts unknown in cable 3.  Does the breaker for this circuit have 2 handles and the handles are tied?  (on Square D &quot;QO&quot; this might look like a double-wide breaker with a single handle).