The Series Three manuals were never converted to electronic format. You'll have to find somebody with a paper copy. The best place to start will be with your local distributor. There was a software package called Logicmaster Three, but GE withdrew it from the market due to problems with it. If you need to modify the program, your best bet is the programmer attached to the Series Three. There was also a Series Three program pack for the portable programmer, but there aren't too many of those around either. All in all, the Series Three was never a successful product. Spend as little time and money as possible maintaining it while you convert to something current.
What do you mean by 'Nematron PC'? Nematron brand labeled operator interface units for GE for several years. Is that what you have?