American Telephone & Telegraph Co. yesterday said its third-quarter net income fell by 5.3 percent, but analysts and company executives were pleased with strong product sales and a 13 percent growth rate.

Net income for the recent quarter was $505 million (47 cents a share), down from $533 million (48 cents) last year. Revenue for the 1987 quarter was $8.47 billion, up slightly from $8.43 billion.

For the nine months just ended, the communications giant earned $1.55 billion ($1.42) on revenue of $24.1 billion, up from 1986 results of $1.31 billion ($1.16) on revenue of $25.56 billion.

AT&T Chairman James E. Olson said the quarter's results were strong and "compare quite favorably with those of a year ago."

"Revenues from long distance were strong and product revenues were higher than a year ago," Olson said.

Year-ago revenue for the nine-month period were hurt by a 26-day strike in June.

Kenneth M. Leon, an analyst at Bear, Stearns & Co., said that this year the company is "stronger than it was 12 months ago." Results point to a 13 percent growth rate, which Leon called "solid." Much of the growth comes from increased product sales, including strong sales of AT&T's central office switching equipment, Leon said.

Bell Atlantic Corp. yesterday announced third-quarter net income of $321.8 million ($1.62 a share), up 8 percent from $297.9 million ($1.49) in the same period of last year. Operating revenue for the third quarter was $2.6 billion, up from $2.5 billion in the same period of last year.

Bell Atlantic's earnings for the nine months ending Sept. 30, 1987, were $955.4 million ($4.80) on revenue of $7.7 billion. In the like period of 1986, it posted earning of $895.2 million ($4.48) on revenue of $7.3 billion.

Bell Atlantic is one of the seven regional telephone companies spun off by AT&T in 1984. It serves six mid-Atlantic states, and owns Chesapeake & Potomac Telephone Cos., which serves the District, Maryland, Virginia and West Virginia