The latest SEC filing from Warren Buffett's Berkshire Hathaway (NYSE:BRK-A) (NYSE:BRK-B) listing the company's stock holdings at the end of the third quarter was released today, and the Oracle of Omaha has made some significant changes to his portfolio.

Last quarter, Berkshire Hathaway held stock in 42 companies worth approximately $89 billion -- and, as of the most recent quarter, that number grew by about 3.5%, to $92 billion, with 43 companies, including one major addition. In fact, Buffett left his total share count of 33 of those companies identical -- but he made major moves with three others.

1. Added a $3.5 billion position in ExxonMobil (NYSE:XOM)
Warren Buffett's biggest move of the quarter came in the form of a $3.5 billion investment in this energy giant. Buffett has long been a fan of large companies with a strong competitive advantage, and this is no exception, as Buffett now holds about 1% of the world's second largest company.

The SEC filing does not say why Buffett added a stake in ExxonMobil, but the discipline and focus it adheres to when it comes to returning capital to shareholders is almost unparalleled. Considering it delivered another solid quarter of results in its most recent earnings release, it should come as no surprise that Buffett would want to invest in this stellar company.

2. Sold $500 million worth of ConocoPhillips (NYSE:COP)
Perhaps in the name of not wanting to be too concentrated in one industry, Buffett sold about half of his stake in ConocoPhillips. The company actually had a very strong third quarter, as its price rose by almost 15%. But when you consider that, in May of 2012, he told Becky Quick at CNBC that he made a "mistake" buying shares of Conoco, you have to wonder if he chose to exit this energy giant following its 33% rise in value over the past year.

3. Unloaded 75% of the GlaxoSmithKline (NYSE:GSK) position
While Buffett had a "small" $73 million stake in the British pharmaceutical company at the end of the second quarter, he unloaded almost all of his holdings in the beleaguered market laggard during the third quarter. Considering that GlaxoSmithKline has been under a wealth of scrutiny lately as a result of allegations of bribery in China, and the patent expiring on its top-selling drug Advair, it may come as no surprise Berkshire Hathaway sold its position. While Berkshire didn't have a major position in GlaxoSmithKline, he likely wasn't thrilled with all the headaches associated with owning the company.