Yes, it would be free over wifi. I have the 3G + wifi and have Calibre set up to deliver news and feeds everyday to my
MyName@kindle.com and as long as the wifi is up I am charged nothing.
I don't automatically send to my DXG because it would cost since the delivery would be via 3G. For the DXG I sideload via USB.
BTW, I've yet to connect my K3 to my PC but I will be doing so soon to load the screensaver hack.