I would say the HK is going to be the easiest to shoot well, since it has a much better trigger than the XD and a more natural POA than the Glock. All three are capable of better accuracy than most shooters. The HK is definitely the best made, but the XD and GLock are still very good weapons.
This is based on my experience, which is an XD 9, Glock 22 .40 and a USP .45. I would take the USP hands-down, even at twice the price. That said, I sold my USP stainless 2 yeas ago to buy a LNIB S&W 1006 (I was that broke). IMO, the 1006 (and 4506) shoot much nicer than any plastic gun I have ever fired. They also have adjustable sights.