Wiring for GFCI with 3 cables
When UL lists the device, they list the instructions along with it.
The reason for the &quot;DO NOT&quot; instruction is because the instructions in the GFCI device are written for the specific case of one cable in; and 0 or 1 cable out.  UL would not approve more complex instructions to handle every case; so the instructions say to get professional help if your case is more complex than the instructions.
The way to wire it is to identify &quot;Always-hot&quot; (which is hot at all times) and true Neutral.  Pigtail &quot;Always-hot&quot; to the LINE brass screw.  Pigtail Neutral to the LINE silver screw.
GFCI devices have the ability to provide GFCI protection to a downline.  If you want to protect downline points-of-use, then for each downline load, hook its Hot wire to LOAD brass screw, and its Neutral wire to the LOAD silver screw.    Only do these in pairs - both hot and neutral of any downline.
Note this can get quite confusing where switches are concerned.