Cardi B. Karwai Tang/WireImage Cardi B is defending her decision to hit the gym days after giving birth to her third baby. The rapper, 31, welcomed her little one on September 7 and shared the happy news via Instagram five days later, writing, โThe prettiest lil thing ๐ธ๐ธ 9/7/24.โ On Sunday, September 15, Cardi took…