I have now ATI 4870 using with Gigabyte motherboard which have ATI 3200 (780G) Chipset. Can i use Both this video outputs to connect three monitors.
Yes, you can. The HD4870 itself can display via 2 ports and the onboard video can display more. Make sure that in the BIOS, you set both the onboard and the discrete as active.
When you booth into windows, you can activate all 3 monitors under display properties.