Missy and Kim are the most successful because they came out at the time when being a female MC meant your music was really played in the US. Nicki came out in the internet age so she is obviously gonna sell more records because she is the only female MC that is currently being promoted by the Media. Nicki is also using the same marketing strategy that was used to promote Beyonce where everybody else was ask about her and knowing that nobody will say anything bad because they are afraid to be labelled as haters.