Q: Siri/iPad3

What do you mean by no home button? Just keep on pressing the home button for 2-3 secs n siri will show up, make sure you turn on siri in the settings>siri turn on.

On your iPad it is the circle button at the bottom that has the square in it, that is your home button.  You need to hold that down and then Siri will pop up on your screen.  If you don't have that then you need to enable Siri, and that you would do from the settings menu, I believe it is in general and then you have the option for siri.

For help with using Siri go to   http://support.apple.com/kb/HT4992