What is the purpose of the three small bumps on the switches for landing lights and what is the history behind it? You'll see a similar design on Boeing, Airbus, and other aircraft.
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.
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?
It’s a tactile shaping so a pilot can recognize the switch just by feel. Allows the aircrew to visually concentrate on more important tasks and simply locate and command the switch by touch alone. Also useful if cockpit lighting is lost at night and you need to locate critical switches.