Any gauge less than 4 foot 8.5 inches (1435 mm) is technically narrow. The "Narrow" gauge in the base mod is only one form of narrow gauge (Minecraft gauge also being an example of narrow gauge) so it does not make sense to use such a general term to label a specific gauge. Since three foot (914 mm) gauge does not have a name used for it (in contrast to Standard or Brunel for example) I believe three foot, 3 foot, or 3ft gauge are the most accurate names for it.