how do i turn off apps in the background on 3g

Very, very few apps run in the background and the iphones os manages those that do.

Double tapping the home button pulls up the recently used apps. These apps are very likely not running in the background.

http://whenwillapple.com/blog/2010/04/19/iphone-os-4-multitasking-explained-agai n/

What software version are you running on your iphone?

You can force quit individual apps.

Press and hold the On/Off Sleep/Wake button for a few seconds until a red slider appears, then press and hold the Home button until the app quits.

If you have a 3G, as opposed to a 3Gs, you can't do it because they're not there. The 3G does not support multitasking. The only thing that can run in the background is the iPod app.

Your iPhone 3G simply does not support multitasking, so you have no need to "turn it off" for individual applications, it's always off.

Continuing though, are you looking to quit applications as a way to regain battery life or quicken the iPhone's performance? Performance issues can be helped by updating from earlier versions of iOS like 4.0, and the like to a later version, like 4.2.1. This can be accomplished by plugging the iPhone into your Mac or PC and selecting "Update" in the iPhone's "Summary" tab in iTunes.

If it's about battery life, Apple has some great tips to extend battery life that can be found right here:

http://www.apple.com/batteries/iphone.html