If they are three eighths of an inch (3/8") thick, they are closer to three scale feet. On the other hand if that was a typo and they are 1/8" thick that'd be about 12 HO scale inches. Actually, 1/8" is closer to eleven HO scale inches but as a rough rule of thumb, it works okay.
Jack of All Trades, Master of Somehttp://www.pawofabear.com
Yup the tiles are 1/8 inch thick and 3/8" square. Guess these will have to be for O scale and not HO scale.