Tele-Communications Inc., the nation’s largest cablecaster, is reaping the benefits of a rate increase, rising subscribers and a big boost in advertising in the third quarter with a tremendous surge in cash flow.As a result of the gains, TCI posted a $ 30 million (6 cents a share) net profit for the third quarter ended Sept. 30. By comparison, the Denver-based company posted a $ 26 million (7 cents) net loss for the same period last year. Cash flow, which is income after costs and expenses but before depreciation and interest expenses, increased 18% to $ 421 million. This gives TCI nearly three times the cash needed to cover its net interest expenses. Revenues were up 11.6% over the comparable quarter to $ 896 million. The results, said Jessica Reif, an analyst with Oppenheimer & Co. were “way above expectations.” This is the second positive quarter for TCI, which has seen earnings drained to pay interest on high-cost debt. Analyst Fred Moran of Donaldson, Lufkin & Jenrette figures that next year’s cash flow won’t increase as fast, but he does expect 8% to 9% revenue growth, 5% to 6% expense growth and an 11% to 12% gain in cash flow. He estimates earnings to be 75 cents to 80 cents a share for 1993. Meanwhile, Comcast posted a $ 52.3 million (39 cents) net loss for the same quarter, compared to a $ 40.3 million (34 cents) net loss for the third quarter of 1991. Revenues were up 24% to $ 221.2 million vs. $ 178.9 million in the comparable quarter. It reported a 28% increase in operating cash flow to $ 98.5 million for the period.