In reply to HYISI.

Songwriting: Katy does more than Beyonce and Rihanna and she's good at it, Beyonce and Rihanna do it sometimes
Singing: They can all sing, Beyonce is more powerful with her singing usually and has better voice control than Rihanna I think but Rihanna's voice might be better for some people
They can all perform fine
And better artist depends on definition of "artist" and sometimes is more of a personal thing