I actually went on this little delve myself a few months ago after re-reading The Three Musketeers for the first time since high school.

Contrary to my assumption, the Traitor's Cross turned out to be a location, rather than a specific cross used for the purposes of crucifixion (although a cross was erected there prior to the French Revolution), the intersection of rue Saint-Honoré and rue de l’Arbre-Sec in Paris' 1st arrondissement, originally named the Tiroir Cross (Drawer Cross, if my handy Google-translator isn't failing me), possibly in reference to the fabric commonly sold there, although other sources claim it was used as a meat market. There was a calvary cross on the site, which certainly may be where the 'Cross' part of the name comes from, but I haven't been able to confirm that. 'Croix du Tiroir' seems to have shifted to 'Croix du Trahoir' (Traitor's Cross) sometime after 1698, the latest period reference I've seen to the original name.

As Dumas suggests, the intersection seems to have been the site of a great deal of bloodshed. There seem to be an infinite number of stories, without historical sources attached to them, about incompetent servants being brought here to have their ears cut off. At least one story, from the April 2, 1825 edition of the The London Literary Gazette and Journal of Belles Lettres, Arts, Sciences, Etc, references a man named Chantepie being "broken upon the wheel" (the Catherine Wheel, a literal wagon wheel a person would be tied to while a torturer smashed their bones with a hammer or other blunt instrument) here on September 26, 1587 during an interrogation about an attempted murder. This may also have been one of the sites where Protestants were executed in 1587 following the Affair of the Placards.

Germain Brice's 1687 Nouvelle Description de la Ville de Paris (New Description of the City of Paris) briefly mentions the location (though he spells it "Cross du Tiroüer" here, it seems to be the same spot). He references a legend about the 6th century Queen Brunhilda, consort to Sigebert I of Austrasia, being dragged to death by a horse here, but also admits that hsiotrians in his time dispute the details of the myth. The 1698 edition includes a longer section on "La Croix du Tiroir" (using that spelling this time), I'm unable to find an available English translation, but at least one Goggle-translated quote from it references criminals being executed here, particularly counterfeiters.