Printronix, a computer printer manufacturer in Irvine, posted net income of $477,000 for the third quarter ended Dec. 25, 1987, a 65% decline from the $1.4 million posted a year earlier. Sales dipped 4%, to $31.7 million from $33 million last year. For the nine-month period, the company reported a net loss of $556,000, contrasted with a profit of $1.8 million for the first nine months of fiscal 1987. Revenue dropped to $90.2 million from $95 million. The quarter and nine months ended Dec. 26, 1986, included a tax credit for current domestic losses, carried back to prior years.