3 days. No grazing. One horse to carry rider, gear and all feed. Only certified (i.e. No seed) feed allowed.
You don't want much
"Technically" it's doable (a horse can get by on less for short periods), but not in a way that most people would want.
Since you'll end at the start point you can (as pointed out) leave the 3rd nights feeding in place there (and make it a good feeding).
Beet pulp is easily digested fiber and worth about 1/3 more it's weight in hay (and it's a no seed feed). You can carry enough to "get by" on for two days, but your horse won't be full and there's always a risk of colic when the digestive tract isn't working normally. Try to feed at least 4 times during the day to keep some food moving throughout the day.
No grazing is the real killer here since grazing stops allow for the gut to be worked thoughout the day as well as providing some level of the nutritional needs.
Of course you can lead the horse allowing it carry more so it meets what it needs. You can hike and backpack what you need.