Why would you want an iPad 3? The iPad 4 is sure to be better!
In other words: waiting for a better device that'll be available later is pointless, because devices are always going to get better, and even at the same price. The exception would be if you have very specific requirements that you know are not met by the current device, but will be met by the next generation device.
I do not know what the iPad 3 will offer just yet. I am not thinking they'll have a USB port, another camera, or anything notable like that though. Perhaps a faster processor or quicker internet speed. I'm very impressed with the improvements in iOS 5, and it's given me quite a bit to play with. I'm happy with my iPad 2.
I suppose I'd have to answer thusly: How soon do you want an iPad? If you want it now, then an iPad 2 is in order. If you can wait, that's fine too!