Okay, I read up on Amazon that if 3G is not available, the Kindle 3 will fall back on EDGE/GPRS. This consumes more battery than running on 3G. This information is on Amazon.
Quote from Amazon:
Long Battery Life - Up to a Month
Battery Life of Up to One Month
A single charge lasts up to one month with wireless off. Keep wireless always on and it lasts for up to ten days. Battery life will vary based on wireless usage, such as shopping the Kindle Store, web browsing, and downloading content. In low-coverage areas or in EDGE/GPRS-only coverage, wireless usage will consume battery power more quickly.
And this is what it says fro Kindle Wifi:
Long Battery Life - Up to a Month
Battery Life of Up to One Month
A single charge lasts up to one month with wireless off. Keep wireless always on and it lasts for up to 3 weeks. Battery life will vary based on wireless usage, such as shopping the Kindle Store, web browsing, and downloading content.
Now, if you have a 3g K3, then it clearly says, that the battery life is UPTO 10 days. With the wifi K3, the battery is rated at 3 weeks with Wifi on. From this, what I deduce is that the 3g part (cellular part) of wireless is the one that consumes the major chunk of the battery. Now, that is if you just leave the wifi on and do NOTHING on it. If you are obviously browsing, then you will get less battery life. In my opinion, what you are getting is perfectly fine. Enjoy your Kindle. To save battery, turn Wifi off, when not needed.
Now, if you are interested in some technical gibberish, let me explain something. When you have a device capable of running on different standards (in this case, 3G and Edge/GPRS), the standard that was developed earlier would drain more battery. Its true with every device.
For further reading
http://en.wikipedia.org/wiki/3G