Hey. Triple Trailers are allowed, but you have to know how to handle it properly otherwise you're liable to be banned as per rule §3 - Save Editing.
§3 - Save Editing
"Causing extreme lag or game crashes by save editing is not permitted. Extreme lag will not occur from duplicate parts unless the amount is truly excessive. If you save edit to create a very long trailer combination but cannot safely handle it, you are liable to be banned for reckless driving and/or blocking and it is recommended that you remove the particular modification(s). If the Game Moderation Management thinks that your modification is not fit for our servers or negatively impacts other people, you are liable to be banned until the modification has been removed."