I would like to know weather the smart cover on a 3G iPad covers the sim-tray completely partially or not at all. A photo would also be really helpful
Only a small portion of the sim tray is really covered by the smart cover. Or, in other words, the smart cover attachment does not hide the sim tray. However, the sim tray pops out straight, parallel to the display, so you cannot remove the sim without detaching the smart cover:
I'll have to update my answer. As you can see, the magnets are just above the sim-tray.
So, no, the smart cover won't cover the sim-tray.