Making WiFi hotspot for 3G network sharing controlled by Arduino
Not a chance, I am afraid (you were right not to buy things yet). Things like the 3G shield and WiFi shield don't work in the way that is needed for a WiFi hotspot like that. You're better off with a Raspberry Pi and a 3G USB dongle and run hostapd along with the other software needed for your project - things like apache, etc.

You may be able to do it with the Linux portion (OpenWRT) of an Arduino Yun with a USB 3G dongle attached, but it would be sluggish. Best to stick to something with enough processing to do what you want, which means a Raspberry Pi or similar.