By Dan Weil Greene estimates his real estate portfolio worth more than $5 billion. Forbes pegs his wealth at $7.2 billion. Real estate investor Jeff Greene bought his first properties in 1977, when he was a student at Harvard Business School. He purchased three apartments in humble Somerville, Mass. for a grand total of $36,900, living in one of them. “I was just looking for free rent,” the 67-year-old Palm Beach, Fla. resident said in an interview with TheStreet. “I wasn’t looking for a career in real estate.” Greene estimates that his real estate portfolio now – primarily apartments in South…