With only 3 hours, subtracting having to be at HOU early enough to clear security and the time it takes you to claim baggage and clear the terminal at IAH, even on a Sunday, would be pushing it for the cheapest, renting a car, and leave you with no real choices but a taxi or car service. Sunday at least leaves the option of driving straight through downtown, shortening the distance, but I would be uncomfortable planning to have time to both pick up and return a rental car.