The third quarter proved to be a rough ride for holders of GSE stock, but the bumps and dips apparently provided opportunities for some large investors to bolster their holdings of Fannie Mae and Freddie Mac shares. Citigroup jumped into the GSE stock market in a big way during the third quarter. The world’s largest financial institution picked up 33.6 million shares of Fannie Mae during the recent period – more than tripling its stake…