UPDATE 1-ON Semiconductor Q3 rev tops Street, sees weak Q4 rev

Nov 3 (Reuters) - Chipmaker ON Semiconductor Corp posted better-than-expected quarterly revenue, but forecast fourth-quarter revenue below market estimates on lower backlog and flat average selling prices, sending its shares down 3 percent in extended trade. The company, which makes audio and power management chips used in mobile phones, medical equipment, cars and electronics, forecast fourth-quarter revenue of $565-$585 million, below analysts' expectations of $593.2 million,