why do you say your hair is so weak? if its just feeling dry because of the bleach job, then its fine to re do something in 3 days. dont wash your hair, let your hairs natural oils build up and then re-dye whatever blonde you want. your at a stage where your hair is platinum so you can basically cover it up with whatever other blonde you want. :) i bleached twice, colored it 5 times in a 2 week period and my hair is doing fine so you will be fine because in your case its only dye youll be using and dye is less damagin then bleach..way less.