American Telephone & Telegraph Co. yesterday reported third-quarter profits of $317 million, capping a nine-month period that its chairman called "more volatile and less predictable than we had anticipated."

Meanwhile, two regional telephone companies, Bell Atlantic Corp. and U S West Inc., also reported strong earnings.

AT&T, which was separated under court supervision from its telephone operating companies in January, said its latest earnings amounted to 28 cents a share and came on revenue of $8.01 billion.

For the first nine months of the year, AT&T reported a profit of $1 billion (91 cents), on revenue of $24.78 billion.

It said the results are not comparable to any other periods because of the Jan. 1 divestiture.

"Our financial results for these first three quarters of 1984 and in particular these last three months have been more volatile and less predicatble than we anticipated," AT&T Chairman Charles L. Brown said in a statement.

"The management of this company remains confident that the current ups and downs in our financial performance will give way over the long term to steadier, more predictable and sustainable earnings improvement," he said.

In the first quarter of this year, AT&T reported earnings of $227 million on revenue of $8.1 billion, while second-quarter earnings were $456 million on revenue of $8.6 billion.

At Philadelphia-based Bell Atlantic, which owns local telephone companies in Pennsylvania, New Jersey, Delaware, Virginia, West Virginia, Maryland, and the District, third-period profits were $249.5 million ($2.54 a share), up slightly from the $246.7 million ($2.53) in the second period.

Operating revenue was $2.01 billion, while operating expenses were $1.47 billion.

"We have sustained our earnings momentum, largely through aggressive cost and force management and better operating efficiencies, and we're well on our way to meeting the 1984 earnings objective we forecasted last November," said Thomas E. Bolger, Bell Atlantic's president and chief operating officer.

During the third quarter, Bell Atlantic acquired Houston-based Telecommunications Specialists Inc. (TSI), which markets and installs telephone switching equipment, and agreed to purchase the Sorbus Service division of Management Assistance Inc.

Denver-based U S Bell reported its third-quarter profit was $228.3 million ($2.39 a share) on revenue of $1.86 billion, up sharply from the second quarter, when profits were $202.6 million on revenue of $1.8 billion.

For the first nine months of 1984, U S Bell, whose subsidiaries include Mountain Bell, Northwestern Bell and Pacific Northwest Bell, had profits of $634.3 million ($6.61) on revenue of $5.4 billion.