| 4 mm to 1 foot|
12 mm (0.472 in)
| 3 ft (914 mm) (Narrow gauge)|
00n3 is the description given to modelling 3 ft (914 mm) narrow gauge railways in 4 mm scale with 12 mm (0.472 in) gauge track. 3 ft (914 mm) prototypes were common in Ireland and the Isle of Man, but the scale is not generally used outside the British Isles. 12 mm (0.472 in) gauge track is the same as that used in TT scale and HOm, so some components used for those scales can be used.
There is no ready-to-run support, so everything must be either built from scratch or made from kits from Worsley Works(C&LR, T&DLR, WCR & others), Branchlines (IoMR, CVR, T&DLR), Backwoods Miniatures (CDJR, CVR, L&BER, C&LR), Parkside Dundas(T&DLR & WCR], Model Engine Works (CVR), Ninelines (CDJR & SR) and Alphagraphix (CDJR, L&LSR, L&BER) amongst others. Kits are available in etched brass and nickel-silver (locos, railcars and some rolling stock), plastic, resin, white metal and card (rolling stock). There are now some 3D printed items of stock available, for example from Model Engine Works, CWRailways & TeBee Models.
00n3 modellers have close relations with other 4 mm narrow gauge modellers, especially 009 modelling.
00n3 layouts that are occasionally exhibited in Britain and Ireland (2015) include Annascaul (Tralee & Dingle Light Railway), Burtonport (Letterkenny & Burtonport Extension Railway), Castlederg (Castlederg & Victoria Bridge Tramway), Letterkenny (County Donegal Railway), Castlefinn (County Donegal Railway), Schull (Schull & Skibbereen Railway), Crosby (Isle of Man Railway), Strabane (County Donegal Railway) and Mannin Middle (Isle of Man Railway inspired layout).
Dingle (Tralee & Dingle Light Railway) is permanently based at the Windmill Museum, Blennerville, Co. Kerry and Donegal at the County Donegal Railway Heritage Centre, Donegal. There is an 00n3 diorama of Boot station at the Ravenglass & Eskdale railway museum at Ravenglass, Cumbria
Outside the British Isles the slightly smaller 1:87 H0 is used. For European 1,000 mm (3 ft 3 3⁄8 in) (metre gauge) prototypes and African / Australian 3 ft 6 in (1,067 mm) gauge railways H0m is used. In America 3 ft (914 mm) gauge railroads are modelled on 10.5 mm (0.413 in) gauge track as HOn3.