Usually a cable breaks in this application due to binding parts/ idler arm on top the deck. Is there a lot of vibration as well -such as from bent spindle pullies? Time to remove the deck and have a look.

I had the exact same problem and I found a bent pulley .I took the pulley off and was able to straighten it by setting it on my table saw cast iron table and tapping on it with a 4 pound hammer.It is running without any vibration now.I am still suspect about the life of the third cable.The way it is attached by going through a cut hole in the steel plate is a poor design.I have read about cable failures that used the same method.