For the nine months to September group earnings were 1.59 rupees per share on total profits of 2.86 billion rupees, down from 3.4 billion rupees a year earlier.
The stock closed at 44.40 rupees, down 40 cents, Wednesday.
Group revenues rose 15 percent to 14.5 billion rupees in the September quarter and operating costs rose 20 percent to 9.88 billion rupees and the firm grew operating profits 8 percent to 4.69 billion rupees. Depreciation was flat at 2.8 billion rupees from a year earlier.
During the quarter the group made 441 million rupees of foreign exchange profits, which allowed it to reverse some earlier forex losses.
Sri Lanka's rupee fell in the first half of 2012, pressured by sterilized foreign exchange sales by the Central Bank. A depreciating currency inflates foreign exchange denominated loans, but also increases rupee revenues from international termination settlements.
Mobitel, the group's mobile unit has made a billion rupee after tax profit in the third quarter, SLT said in a statement.
"Consistent Growth in operating profits in 2012 demonstrates our strength, resilience and aligned growth strategy of our group of companies to deliver strong results," Chairman Nimal Welgama said in a statement.
He said the SLT will continue to invest in expansion.
Group cashflow statements showed investment of 10.2 billion rupees in new plant and equipment.