One of the nice things about the vintage bikes is that they often have clamp-on front derailleurs. This allows a fair amount of vertical adjustment, either up or down as needed.
The vintage front and rear derailleurs may or may not play nice with a triple, but they may do fine with a double MTB crankset with appropriate not so wide bottom bracket.
What do you need for top-end gearing?
Here is a Suntour Octalink 22/36 for not too much.
https://www.universalcycles.com/shop...s.php?id=72176
You could polish it to silver if you wished.
Of course, there are triple Front derailleurs and vintage long cage rear derailleurs if you wish to go with the triple.