It depends how you play the cuck path. I have a save where Chad Ian cheats on Lena with Cherry, Alison, Cindy, Minerva, Emma and had the threesome with her and Louise, while Lena has fucked only him and Louise, but still gets onto the cuck route. He's not behind anyone on this path.
Technically, they weren't cheating early on, because before Chapter 9 they were only hooking up. They didn't become boyfriend and girlfriend until their Chapter 9, talk in the park. So that immediately rules out Cherry and Emma, because Ian can't have sex with them and be Lena's boyfriend. Also when you're on the cuck route, it's impossible for Ian to have the threesome with Louise ( that must have been on another route). Yes, it is possible for Ian to be on the cuck route, and still cheat on Lena. But the question is more why would you? If Ian is cheating on Lena, then that version of him would never realistically be on the cuck route. Yeah you can do it, but it doesn't really make a lot of sense. Cucks tend to be unwaveringly faithful to their partners.
On my playthrough of the cuck route, I don't allow Ian to have sex with anyone but Lena. He's only really interested in her. He flirted a bit with Emma, Cindy and Alison early on, but it never went anywhere. And after they have sex for the first time, he becomes totally devoted to Lena.