How to make my parents agree on getting a dog?

I really really realllyyyy want a dog! like no joke..i told my mom and she wants one too but my step dad not so much. he loves animals but mostly cats..we have 3 cats :/ my mom says when some of the animals go we can get a dog (2 of my cats aree 14) but i just wish i could convince them to agree to get a dog soon! please help..thanks