I have a folder with 110 rear spacing, Can I install a Sturmey Archer 3 speed ?
Yes, but you'll have to get a hold of one of the hubs made for Brompton and relace it to a new wheel, unless you're looking for a ETRTO 16" wheel. Bike shops specializing in Bromptons are better sources to ask.
If you're in the U.S., the west coast Brompton dealer in Palo Alto won't help you - I've tried.
You might be refering to me, Mr. Haury. I sucessfully converted my Boardwalk S1 to a three speed some time ago. It can be done. I do not know how it is done except that my bike already had a horizontal rear dropouts (for easier chain tensioning), steel frame (can be expanded or contracted to fit different hubs), and I knew that I wanted a simple rather old fashioned thumbshifter on my handlebars, uncluttered look as in ss and fixies, and operator friendly. My LBS did the conversion and I have never been happier with the bike.