It’s not yet known if immunocompromised individuals will need both a third dose and a booster. The third dose will help you build more immunity, as your immunocompromised condition may have prevented a good immune response with the first two doses. So it’s recommended that you get the third dose now and not wait for the booster.
Pfizer booster shots are recommended by the Centers for Disease Control and Prevention for certain eligible individuals. Both Moderna and Pfizer are authorized for a third dose 28 days after receiving your second dose.