What is the significance of numbers (2,3) mentioned in SOT23?
While some packages do have numbers that mean something, the TO, DO, and SOT series are simply serial. SOT-23 is just package number 23 in the JEDEC SOT specifications. There's no significance to the numbers beyond that.

You may occasionally see SOT-23-3 or SOT-23-5 or SOT-23-6; these do have significance, as the last digit is the number of pins on the package. Sometimes SOT-23-5 gets referred to as SOT-25, as well, but this is generally considered incorrect. Similarly for SOT-23-6 being called SOT-26.