All tortoises grow at different rates, and the way each keeper has his tortoises set up, feeds, heat, lights, etc. is all factored into the mix.
I would say that a three year old sulcata should probably be in the 6 to 8 SCL inch ball park. Of course, some are bigger, some are smaller.
The mediterranean species are pretty easy to guess because they all fall into the Jackson's Ratio, but sulcatas are just too different from each other.
(SCL means the straight line measurement from the front of the shell to the back of the shell)