TT a Triang invention, TT standing for "Table Top" but because of the scale of 3 mm to the foot became known as TT3. Apart from Triang of course Kitmaster made some TT3 models, the rebuilt Royal Scot, and two corridor coaches. The Scot sold initially at 6s11d, the coaches a shilling cheaper. Bec and Gem also made kits. It suffered from the same problems as 4mm to the foot and 16.5mm track, only on this smaller scale the disproportions were more obvious.
I wouldn't have thought there was much of a market for TT3, but there is of course a very active society, the Three Millimetre Society, who would disagree with me