The bike is almost 40 years old and if these are the original wheels the spokes are probably at the end of their useful lifespan... 3 broken spokes is an indication that they are suffering from fatigue, either through improper tension or simply from old age.

If the hub and rim are in good condition and you want to stay wiith original parts I would rebuild or have have the wheel rebuilt with new stainless spokes.

Building a wheel from old parts is harder than using new parts as you will be dealing with parts that are usually less then perfect but good results can be attained if the parts are sound and not worn.

A tension meter is a useful tool, especially when wheel building is something you do on an occasional basis and do not have a good sense of what proper spoke tension should be.