How can I install viber in my iPhone 3. 8 GB?

First, you have to be more specific about your problem. But from the info that you have given here, I'd say that all you need is YOUR app store account, and internet connection.

you'll open the store app from your phone, search for viber. Then when it comes up, just tap install.
it'll ask you then about your store account password ... provide it and wait for it to install.

There is no iPhone 3. Had to be the 3G you are referring to since the 3GS can update to the latest iOS version.

Nothing you could do. It requires a higher iOS version than the 3G is capable of running.