Novell Inc.'s third-quarter sales ending July 30 increased 51 percent over the same period in 1987, the company says.

Novell, a manufacturer of computer networking products, said its third-quarter sales were $73.39 million, compared with $48.65 million for the corresponding period in 1987.Third-quarter net income rose 106 percent to $10.84 million compared with net income of $5.26 million for the third quarter of the previous year.

Novell said net sales for the first nine months of 1988 increased 56 percent to $195.54 million, compared with $125.52 million for the similar period in 1987.

Net income for the first nine months of 1988 increased 77 percent to $24.84 million, compared with $14.05 million in 1987.

Net income per share was 39 cents for the third quarter and 90 cents for the first nine months of 1988, versus 20 cents and 55 cents for the same periods in 1987.