This is a really good question! “I wouldn’t believe it” indicates that this is something you believe in the present. “if I hadn’t seen it with my own eyes” indicates that that you saw something in the past. “I wouldn’t believe it if I hadn’t seen it with my own eyes” indicates that you saw something in the past which has affected what you believe in the present. In this example the past thing you saw is relevant to the present moment. “I wouldn’t have believed it if I hadn’t seen it with my own eyes” is talking firmly about a past event, the emphasis is on what you saw and believed in the past with less relevance to the present moment. I would argue that the first example is incorrect. If you want to emphasize that the thing you are seeing right now in the present moment is unbelievable I would say: “I wouldn’t believe it if I wasn’t seeing it with my own eyes” Best of luck mastering conditionals!