Hi Luckyjo!
Ideally, you would not want to sit with your back to the Three Killings. If you are facing Southwest when in your office, your back is actually to the Northeast, so it shouldn't be a major issue. If you do feel vulnerable, you might want to consider putting something that feels protective either on your desk or flanking it, that faces toward the North. I've had several people use either one of the Archangels or an animal that feels very special and protective to them.
If in doubt, you can also muscle test to see what the energetic impact would be of any options you are considering.
One other thing I'd like to mention is that any time you have some challenging stars on an area and still have to use it, you might consider putting the Diamond Feng Shui Diamond (page 31 of your Level Three Manual) up in that area. That graphic carries a very positive energy and if you put it up in that room, or over a door, bed or desk, it can have a really great effect on the overall feeling of the space and what happens there.
All the best!
Wendy
