At the moment I’m actually training Chest/Shoulders/Tris most weeks, def not ideal, but I’m only getting to the gym 3 times a week. I don’t think doing either Shoulders or Tris after chest will cause over training, as long as you bare it in mind and lower your volume. In fact putting them together may have the opposite effect and prevent over training. For example, if you train Chest, Shoulders and Tries all on separate days, your Tris are being worked all 3 times and Shoulders twice.
Given the choice, I would put Tris with Chest together, as I like to train Shoulders heavy. I think the best thing is to change it around every so often and just see how it feels.