There are various ways of showing what people are modelling in Narrow Gauge... The usual American way is to show the scale first - O... Followed by "n" for narrow gauge... And finally the gauge - 3 in this case for 3ft gauge...
The 30 in On30 is for 30 inches representing 2' 6" gauge...
O scale in America is 1:48, in the UK its 1:43.5 and in Europe 1:45!
Hadn't heard of it before, but a quick Google shows it as using a track gauge of 19.05mm...
In HO scale 3ft NG is quite big, HOn3 uses 10.5mm gauge and has plenty of commercial support, you'll have to do some searching yourself if you want to know if there's any support for On3 though!
http://en.wikipedia.org/wiki/Modelling_ ... _railroads