FWIW, I leave positive feedback for all of my buyers as soon as they pay and make absolutely no mention of feedback or DSRs on the packing slips I enclose with their items yet my buyer feedback is averaging around 77% (or about 3 feedbacks for every 4 sales) with some buyers taking up to a month before leaving feedback, which I put down to the reminder that eBay automatically sends, so try not to feel too disheartened as I'm sure it won't be too long before you receive the 3 you need to see your DSRs.