3-way GE Smart Switch installation
OK, so here's what we know about 3 cables
(and can guess about the 4th).  Note that &quot;color coding&quot; is not happening here and the color codes will be a chaotic rainbow, with no consistency whatsoever. This situation is why I own 10 colors of tape.
The only consistent color rule is if neutral is present, it must be on white.  And that's the end of the color rules (aside from stuff about grounds and 3-phase industrial power).
SUPPLY cable and ONWARD cables
Both are usually /2 cables but onward might be /3. The supply cable comes from the power source.  The onward cable chains power onward to the next outlet.  Both of them have the same function and color codes (by necessity, unless they're /3, then all bets are off).

Neutral on White
Always-Hot on black in /2, and ??? in /3.

Spur to the other 3-way switch
This will be a /3 cable by necessity.  Black White Red.   There is no standard whatsoever for colors. Neutral is not present in this cable.  Now, travelers can be identified by the fact that they go to the brass screws on each switch. Both travelers are interchangeable (one is hot and one is not; throwing the switch swaps them).  These are the wire functions:

Traveler 1: color ???
Traveler 2: color ???
Common (could be always-hot; could be switched-hot): color ???

Note how in 3-way switches, colors are completely NOT standardized. At all.  This is why I'm all about the colored tape: but don't bother, we're about to re-define all 3 wires.
Spur to the fan and light
Since both fan and light are controlled by switches here,  we know that there must be 2 switched hots.  By the nature of things there must also be neutral.  So it must be a /3 cable, and must have these wire functions:

Neutral: White
Switched Hot for Light: ???
Switched Hot for Fan: ???

Conventions
Code does not require color choices (except for white and of course grounds).  However, better installers tend to favor certain colors when feasible, and as said I'll &quot;force this along&quot; by remarking with colored tape.  The preferential colors are:

Always-hot: Black
Switched-hot:  Red
Switched-hot (alternate): Blue
3-way travelers: both yellow (that's a Harperism not a wide practice, sadly).

You can't count on that applying here, but you can favor it going forward.
These GE smart switches
The &quot;master&quot; GE smart switch requires 4 wires aside from ground:

Supply hot
Neutral
Switched-hot to the load
Comm/power line to the the &quot;remote&quot; smart switch

The &quot;remote&quot; smart switch NEEDS 2 wires:

Neutral
Comm/power line to the &quot;master&quot; smart switch

So you'll have to put the smart-switch master in the busy box.  As for the smart-switch remote, you'll completely redefine all 3 wires in the 3-way spur cable to be a) Neutral, b) Comm/power line and c) spare (hold black out as a spare so you can use it for always-hot in the future).
That's as much as I can tell you without knowing more about the box wiring.