Can I add 3rd chainring to a an old Sugino double?
Check out the IRD Defiant wide range double crankset http://store.somafab.com/irdwicorodoc.html) It has a 94bcd which allows it to run 46x30 rings. Combine that with a 12-27 cassette or freewheel and you'll have plenty of range. 

Going to a triple presents a few problems:


They don't shift as well as a double.
You have a short cage rear derailleur so max rear cog is 27, maybe 30 teeth. Definitely won't handle 34 teeth.
Triple cranks require triple compatible front derailleurs. 
A double is simpler and lighter. 
The Interloc will look a lot nicer on your 80s era bike. 
A triple will usually require a longer bottom bracket spindle than a double.