Is whatsapp is available for iPhone 2G with iOS v3.1.3 Is whatsapp available for iPhone 2G with iOS 3.1.3?
Unfortunately, no.

https://itunes.apple.com/us/app/whatsapp-messenger/id310633997


  Compatibility: Requires iOS 4.3 or later. Compatible with iPhone.
  This app is optimized for iPhone 5.