[ghost1001]

Another option...

I remeber back when i was a bike mechanic there was a bolt on 3rd ring.....like a really tiny chainring that you could climb mounteverest with....

Now if you can locate this your front shifter would most likely work with it, there are 2 limit srews on the front derailleur if you open up one you should be able to drop to the third ring....it may take some tweaking but i am 99% sure it will work.

On an older bike a 3 ring crank should bolt right up and shift fine especially with down tube shifter, they do not index on the front rings it is a friction set up and again with the limit screws you should be able to get to all 3 with no problem.