I think the best way to convince your parents is to show them how responsible you are with your current pets. If you don't walk the dog often, you could walk it everyday and keep it up. If you don't clean the litter box often, do that. Maybe cleaning the hamster's cage if you don't do it too often. If you could keep up with taking care of all the pets, your parents should see that you're ready for another addition to the family.