Is that Okey that I can tell my kids about their dad's infidelity?

I divorced 3 years ago and got 50/50 custody. I have 3 boys age 17,15 and 10. Their father was cheater, liar and very manipulative. The boys met their dad's girlfriend a year ago and they like her. The boys believe their dad met her last year. That's not true. Their dad was involved with her 5 years ago and this triggered off our divorce. I'm getting tired of hearing of HER from the boys. I just want to spit out.........but I don't know how the truth of their dad's infidelity affect the boys.