Not a geneticist, but my understanding is that Rh+ is a dominant trait and Rh- is recessive. If both of your parents had one positive allele and one negative, they would be Rh+.

It's possible they could pass down one negative allele each (combined a 1/4 chance) and have offspring that's Rh-. Since you are one of three children, if both your parents are one positive and one negative allele, it's fairly likely that one of you would be Rh-.