Sounds more like riding beyond your fitness than a nutrition issue. With 530 Cals in 3 hrs you could eat a little more but it likely wouldn't have helped.
How many hours are you riding during the week? A powermeter would help determine how big the effort was relative to other rides.
It could be as simple as you were just going too hard during the 3 hrs. As your fitness improves your body will utilize a higher % of fat for it's energy needs. If you're riding close to your threshold most of your energy will be coming from carbs and given that you're a pretty big rider it's not unexpected that you could exhaust your available carbs in a little over 3 hrs.