Can I add 3rd chainring to a an old Sugino double? I have a vintage (165mm-Crnk) Sugino 40/48 on here now, but am looking to add some gear range for up hill with med-sm load. I have a Tiagra short cage RD and  a Sora double FD. I am friction shifting. I'm considering doing a chain ring like 26/36/48  with an 11-34  freewheel  which would give me adequate range, I believe 20-117 ish. 

Can I add a chainring to my set? Mine is 110mm 5 arm which sems like it should be replaceable with say a Shimano. I was also thinking I could just get a smaller maybe 36 tooth and call it a day with 2 rings.  

My understanding so far is that my Tiagra rear derailleur has maximum of 28T. I've heard of people friction shifting using a double FD for triple front rings. Has anyone seen this?
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.