Congrats on the ride

WRT a third bottle, I have used four approaches.

1. Camelback. Too much weight and sweat on my back. Rejected.
2. Xlab 400 carbon wing that attaches to the seat rails and allows two bottle cages. I like this solution and used it on PBP
3. Put a third bottle in your handlebar bag. This only works when I carry a HB bag and if the weather is cold or rainy, there is no room for a waterbottle but then again, who needs much fluid in cold/rain.
4. Carry the third bottle in a rear jersey pocket. I only use this approach if the distance to the first control is quite far and I intend to jettison the bottle (not littering of course).

Once past the first control, they are rarely more 40-50 miles apart.