What is the purpose of the three small bumps on the switches for landing lights?
The purpose is to make it distinguishable by touch or visual as explained in other answers and comments. And because generally, there are two main landing gear and a nose landing gear, it has been designed to be more like landing gears.

Edited:

There are two main questions about landing light switches in Aviation Regulation as it reads in FAR 23.1301 which is required to get AIRWORTHINESS COMPLIANCE certificate.


  Is the landing light switch located so as to be readily
  accessible to the pilot?
  
  Is the landing light switch adequately labeled as to operation
  and function performed?