The whole door thing is a tricky problem… I tried to see how I could make it look natural; tell me what you think. The last frame (when the door is almost completely open) is optional, since you only wanted 3 frames total. If you want the door to open as far as it is in frame 4, but still want 3 frames, you could probably just skip the 3rd frame. (The open space is gray, since I didn’t know what to put behind the door.)
<br><br>
<img src="door1.gif">
<br><br>
The main thing is that as the door opens, its topmost edge should be angled upwards. 
<br><br>
Hope this helps.
<br><br>
Ian