Using iptbales to share 3g network over wifi on android

*I want to share the 3g internet connection of the android device through wifi but in a special manner described below:
*my chain is
the android connected to 2networks (the sim 3g+ the AP wifi) -> Access Point -> PC connected to AP over ethernet
[IMG="network diagram"]s7.postimg.org/4407ui9iz/Document_2.png[/IMG]
*so my solution plan was no more than 3 steps:
1-connect the android to the 3g network
2-connect the android to the access point wifi
3-use iptables to forward the traffic coming from "the PC through the AP to the android wifi interface" via "the 3g interface"
*but the problem is:
When i connect 3g then trying to do step 2 means connect the android to the AP the 3G connection disconnect and the 3g interface goes down automatically hence i can't connect wifi and 3g simultaneously
*my questions:
How i can connect wifi and 3g simultaneously