The Sturmey 12T S3X cog is part number HSL886 for 1/8 chain and HSL893 for 3/32 chain. You'll also need the special-for-12T HMW536 (2mm thick) or HMW535 (3mm thick) spacers.
48Tx12T on 700Cx23 wheels/tires will give you fixed gears of 66, 79 and 105 gear inches.