It's true that prednisone has some serious side effects. The thing is that if you are feeling so bad, prednisone will help you stand on your feet within 2-3 days. When I took it it was for almost 4 months and it was the first time I felt hungry again, no pain in my stomach and as far as energy levels, I could participate in the Olympics!!!!!
Prednisone and steroids in general are not long term treatments, but they can help you get out of a flare within days. The side effects I experienced was moon face which was nothing serious because I was very carefull with salt and sugar, and mood swings. But I would rather have these side effects, than running to the bathroom 10 times a day, feeling nauseous and sick.
The dosage your doctor proposed is not very high and the period of time is very short (3 weeks), so I really don't think you will have serious side effects. Of course the decision is yours, but sometime we have to weigh the situation.
Hope this helps,