Micro D Inc. posted net income of $1.7 million for the third quarter, 21% higher than the $1.4 million reported for the period in 1987. Sales rose 57% to $134.6 million from $85.8 million. For the first 9 months, net income increased 40%, to $5.5 million from $3.9 million for the comparable period last year. Net sales rose 56%, to $380 million from $244.4 million. Micro D, which is based in Santa Ana, is a nationwide distributor of microcomputer hardware, software, peripherals and accessories.