subsubsection with letter

So, I am writing a RMD file and and I want my subsubsections to be fronted by letters and not numerics. I can do this by hand but I want to use number_sections: true for conviniance. Hence, I tried rewritting the subsubsection command with:

renewcommand{subsubsection}{Alph{subsubsection}}

I imported the rewritten funciton it in my RMD file. However, this is without success. What am I doing wrong do I need to specify further arguments in my function?

Any help is warmly welcome!