I haven’t done one of these but I recently did a gravel grinder on my Dads 1980 colnago. It had some steep climbs. The original gearing is a 52/42 up front and 14-25 rear casette with 5 cogs. A number of companies used to make triplizer rings to add a third ring up front but no companies make them anymore.

I found this guy who machines them :
http://www.redclovercomponents.com/about-triplizers.html

I got his triplizer and had to try a few different bottom brackets to get the spindle length right and put a longer cage on the rear derailleur fabricated by soma for that purpose this era bog bikes. I put a 24 on the front :) so there was a lot of slack in the chain but the longer soma cage took up the slack.

It worked. It still needs a little work to shift well to the small chainring. I got some spacers for the bottom bracket that should take care of it I think.

I had a blast on the gravel grinder - did the dirt diggler in North Carolina. Had some steep stuff and even 2 miles of single track at the end. I was definitely the only guy with shifters in the Downtube.

It worked though. Ended up 6/15 in my age group just messing around having fun