None of us are able to tell you what you can and can't do, Goldfinch. Te question you have to ask yourself is whether your fatigue is manageable if you pay close attention to pre- and post-ride nutrition, and to getting more sleep, or whether it is cumulative and there is a serous danger of you simply being unable to complete day 3. If the latter, my suggestion would be to miss the ride. You are doing this for fun, after all, and your enjoyment will be severely compromised if you go into the event in a state of high anxiety about your ability to do it.

On the other side of the equation is the sense of achievement you will have if you are able to finish, of course.

Irrespective of what decision you make, the depth of your fatigue after back-to-back rides is surprising given how many miles you are doing. Might be worth thinking about the amount of iron in your diet?