What is meant by this theory that there are only 3 possible kinds of cognitive loads?
The source you mention is a web blog about computer and office work. The blog doesn't describe a 'theory', not even a hypothesis for that matter, it just outlines a starting point for a popular scientific blog on cognitive load in everyday life. It all depends on the task what adds mental load. Every input, let it be acoustic (e.g. speech), visual (e.g. computer tasks), touch or internal mental processes can all increase cognitive load. 

I can recommend reading the wiki page on cognitive load as a starting point to better understand different aspects of cognitive load theory.