Katy and Gaga's career are way more impressive seeing as they're both under 3 albums and Rihanna has released 7 albums already and countless of features.
Gaga also has a few more side albums (remixes, holiday EP) while katy only has one.
Take away the side-albums and consider the fact that Katy's re-release sales were included in Teenage Dream's overall total and it still a one sided match though.