Apple did not support iphone 3Gs to have siri,and only support on iphone 4s.
But I did not want to buy a new one.
How can I install siri on 3Gs?

You need to puzzle yourself the frameworks and needed files together, it requires a lot of trial and error to get it going on a non 4s device as well as a valid "token" from a 4s.

Apparently it can be done but it is not as simple as hacking the iphone and installing the app. Siri needs to communicate to a server and will not do it if the phone is not the 4S. I've also read that iphone 4 and below are not powerful enough to be reliable that is why Siri is not available for these devices.

They are, we ran it on an iPod recently 3Gs should still be capable of doing that. Still you need to get all the frameworks stuffed together and for other devices you might need to tinker with drivers. You need to fight lag and other problems before you can sit back with statisfaction.

I don't think you can just go in and pick out certain features of the upgrade that you want. They all kind of work together.