Is it possible to Share Android phone 3G internet connection to multiple clients (tabs/computers) via bluetooth
There's the bluetooth PAN profile which allows just that.
It is included with CM7+ and from Android 4.0 onwards (or already 3.0?). Bluetooth PAN's host (the device that provides the internet) acts as a router which does NAT + issue dhcp adresses for its clients. 

There are also  Bluetooth DUN profile apps that allow this (they provide a virtual GSM modem and may also allow for more than 1 connection at a time):
See host+client BlueDUN apps from Mirko Solazzi