Girls, my blood type is 3 positive, my husband's is 2 positive. I gave…

Girls, my blood type is 3 positive, my husband's is 2 positive. I gave birth to twins last month. They have a blood type of 3, and both have a negative Rh factor. I read online that ++ will always be +. Is this true? Or could they be negative? Could there have been an error at the hospital? Is a 3 negative blood group dangerous for girls? What are the features? What should we consider with a negative Rh factor?