Boeing's Co.'s third-quarter earnings topped those of a year ago by 43.2 percent the aircraft manufacturer said yesterday.

Net income was $132.6 million ($2.07 a share) compared with $92.6 million ($1.44) in the 1978 third quarter.

Boeing Chairman T. A. Wilson cited an increase in sales for the period to $2.1 billion from $1.45 billion, "continued favorable trends on commercial jet transport programs," higher interest income and lower federal income taxes as reasons for the improved profits.

Boeing reported nine-month earnings of $360.8 million ($5.63) on sales of $5.8 billion. Comparable figures for 1978 were sales of $3.8 billion and earnings of $215.3 million ($3.36).

Ralston Purina Co. announced yesterday that one-time charges for anticipated divestiture losses caused its fiscal year earnings to fall 19.6 percent from last year's level.

Earnings for the year ended Sept. 30 were $128.1 million ($1.19 a share) on sales of $4.6 billion compared with earnings last year of $154.6 million ($1.44) on sales of $4.07 billion.

R. Hal Dean, chairman and chief exucutive officer, said this year's earnings included a write-off of $21.5 million (20 cents) to cover anticipated losses on the sale of the company's Green Thumb floriculture operations and 232 Jack in the Box restaurants and related facilities.

Dean said fourth-quarter earnings were $27.5 million (26 cents) on sales of $1.23 billion compared with earnings of $34.2 million (32 cents) on sales of $1.03 billion in the same period last year.

Greyhound Corp. earned 93 cents a share in the third quarter, up from 16 cents a year ago when the company had a large write-off on the closing of certain facilities of the Armour meat and food division.

Net income was $40.71 million on sales of $1.189 billion compared with $7.11 million on $1.116 billion.

Nine-month profits were $85.30 million ($1.94 a share) on sales of $3.5 billion against $34.16 million (78 cents) a year ago on sales of $3.196 billion.