For a regular spending strategy?
CSP will get you 2x on dining and restaurants, CFU gets you 1.5x everywhere else. I'm not sure what you're looking for in a 3rd card.... You could get Citi Premier as suggested, and then you'll earn 3x TY points on travel and gas, but there are few ways you can utilize Chase UR and Citi TY together (Singapore and Virgin Atlantic I believe are the only two shared partners). An airline card could be worthwhile for free domestic checked bag, priority boarding, etc., if you fly enough to justify the annual fee (usually ~$100).