I am there the same time (and same hotel) for the half marathon weekend. The annual pass discount I got for the weekend was very good, but I booked in January. The discount was not as much when I booked a room about two months out for July, but I don't know if that was related to timing or season differences.
It certainly sounds like you would come out ahead with one Infinity pass and then using the perks to purchase tickets for the other two in the party (not to mention the discounts on food and merchandise).
If this is your first visit and you are used to WDW, lower your expectations about the food. And on that subject, As far as I know, the breakfast buffet in the hotel is the same now that you pay for it than when it was included in the room price, so there is no way it is worth the cost (it is not a full buffet like in a WDW restaurant - just toast, cereal, runny eggs, bacon and some veggies and cheeses).
Don't hesitate to contact me if you have more questions.