Shingles for the past 3 days, how long will it last? and is there any complications could happen?

Was shingles professionally diagnosed? Being treated with an anti-herpes drug like valacyclovir and maybe a steroid like prednisone? If not, see or speak with your doctor. Even with maximum treatment, 3 days is too soon for improvement; it should be better in a week or so, up to 3-4 weeks for complete healing. Complications depend on location (e.g. eyes); sometimes prolonged pain after healing.