I would drop the United MP card and get the Chase Sapphire Preferred. That way you can leverage the Ink/Freedom/CSP combo and get a nice suite of 2x and 5x bonus categories. The Freedom is valuable when used with the Ink or CSP because you can get 5x points on the bonus categories, which is pretty crazy (and last year it was 10 on amazon).

I'd add the Amex PRG when there's a decent sign-up bonus (50K), you'll get the no foreign transaction fee benefit as well as 3x on airfare and 2x on dining/gas/groceries in the US. The CSP will also give you no FTF but you're subject to getting screwed on DCC and Amex is immune to DCC so you really need an Amex if you're traveling intl.

You can dump either the Amex Blue assuming it won't mess with your credit (is it your oldest card by 10 years, etc)?