I thought this as well to be honest with you. Especially using the mobile version as I and others have been doing. Perhaps the system does not update the fact all three FP's have been used and allows us to book new ones thinking the others have not been used.
You would think a few hours would be enough time for even a mobile application to update what has and has not been used. But who am I to argue as it is working the way I like it for now
Also, today is the day for the big test everyone wants to see. I have 6 days of FP's booked, plus I just booked 3 more FP's for today. Upon using the 3 for today, I am going to book the 7th day at the end of the 6th day and then try and book 3 more FP's for today in the same fashion as I would if I only had 3 for today.
I'll report back how it goes this evening or tomorrow morning Depending on how late we are out in the parks this evening.