Here is my $.02

Unless there was external damage, spokes break because of fatigue. Spokes don't fatigue individually but the whole set more or less fatigues evenly. Some maybe a little bit faster depending on their relative tension. If you have had 3 spokes break so far I would bet the others aren't too far behind.

A rule of thumb is when 4 spokes break on a wheel it is time to rebuild it.

It's $20 for set of 2mm stainless straight-gauge spokes. Why mess around with 30-year old spokes that you don't even know the history of? If this is just a cruiser for occasional riding then maybe let it go for now with a few new spokes. If you plan on riding the bike regularly it's probably a good idea to rebuild the wheels.