Actually, its the opposite. Its based on how old they'll be during the trip, not when you book the trip.
Maybe you are thinking of the Disney policy that if they are 2 at the beginning of the trip, then that is the age Disney goes by for the whole trip in regards to tickets even if they'll be 3 at some point during the trip. Same with age 9 at the beginning of the trip. Then you can buy the child ticket and not the adult ticket even if they will be 10 half way through the trip.
I was going to Disney last year (August) for
free dining and the man asked me what age they will be on the trip and it was 6 and 10 (even though she was 9 when I called) so I had to pay the adult price for her ticket but at least it was free dining.
Some people book their trip 2 years in advance. If they book and their child is 2 at the time but during the trip they'll be 4; a 4 year old needs a ticket to enter and it would be quite obvious that they do need a ticket. Booking early does not lock in the age they are at the time of booking.
Same thing with making ADRs; you should be telling them their age at the time of the trip, not their age at the time of booking.
I'd just book the day before she turns 3 and then there is no issue and no worries. I wouldn't try and trick or cheat Disney though to save a few bucks.