3mm is alive and well down in sunny West Wales - we currently have three layouts built using 12mm peco H0 track, and quite a bit of old Triang stock. As 3mm John says, the 3mm Society is a good one to join - access to plenty of kits and parts.
The biggest challenge with 3mm (or TT) modelling is the scenery and trackside. Most things, particularly buildings have to be scratch-built, though you can get away in some cases with H0 figures and vehicles, if you tuck them amongst other things!
Best of luck with it.
Shaun.