Martin Marietta Corp. yesterday said its earnings increased 5 percent in the third quarter, rising to $55.6 million ($1.02 per share) from $53.2 million (96 cents) in the corresponding period a year earlier.

The Bethesda defense contractor reported that revenue rose to $1.31 billion from $1.17 billion a year.

For the nine months ended Sept. 30, earnings totaled $171.2 million ($3.13) compared with $158.9 million ($2.88) in the comparable period last year. Revenue for the period rose to $3.82 billion from $3.48 billion.

MCI Communications Corp. yesterday said earnings rose 22 percent in the third quarter ended Sept. 30 to $22 million (8 cents a share) from $18 million (6 cents) in the same quarter last year. The third quarter of 1987 includes a pretax gain of $2 million from antitrust settlements.

The company said revenue rose to $994 million from $910 million a year earlier.

For the nine months, MCI posted profits of $55 million (19 cents) on revenue of $2.9 billion, up slightly from profits of $54 million (20 cents) on revenue of $2.7 billion for the same period a year ago. Profits for the same period last year included a pretax gain of $65 million from the sale of MCI Airsignal, a paging and cellular telephone subsidiary, and an aftertax extraordinary loss of $17 million from the early redemption of some subordinated notes.