How to install ios 4.3 apps on iphone 3g?

Just how can i install ios 4.3 apps on my iphone 3g, which runs ios 4.2.1.
Is there any way that i can force install ios 4.3 on my iphone, or can i install ios 4.3 or even ios 5 apps on my iphone, either force install, or finding an earlier version of the app, in that case can you give a good website of apps earlier versions, for ios 4.2.1.
PS: do actually a step by step, not a Small confusing answer, thanks.

Just go to settings and open general and choose software update

Nope. Nothing.

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