While all three feel the same, the XD striker is fully cocked. Pulling the trigger performs a "single action". It releases the striker. The Glock and M&P have the striker only partially cocked. Pulling the trigger performs a "double action". It pulls the striker the rest of the way back, then releases the striker.
Actually, the M&P's trigger is designed as a single-action-only system. When the trigger is pulled, the trigger bar's movement rotates the sear, which releases the already fully-cocked striker (much like the XD's system). This is completely unlike the Glock's trigger system, wherein the "sear" is part of the trigger bar and pushes the striker straight back the rest of the way until it is fully cocked, and then the connector pulls the trigger bar and "sear" downward, releasing the striker.
Now here is where things get a little tricky. For some strange and unnecessary reason, the M&P's sear has a ramp on its striker engagement area that causes it to cam the striker back very slightly before releasing it. This makes the M&P's trigger pull heavier, stagier, creepier, and maybe even somewhat grittier than it otherwise would be. This is why the M&P's trigger is said to be so easy to improve--just about anybody can grind off that ramp on the sear (and radius the sharp edges on the striker block plunger), with the result being quite a superb trigger (with the potential to be even better in the hands of a competent gunsmith, or with drop-in aftermarket parts such as the famous Apex Tactical hard sear).
So if the M&P's trigger system is inherently good in terms of trigger pull, then why would S&W deliberately screw it up so badly? One plausible theory is that they wanted the ATF to classify the M&P as a double-action-only pistol like they do the Glock (erroneously, in my opinion, but that's what they call it), and the extraneous ramp on the sear that nudges the striker is how S&W managed to pull it off. Since the striker moves back (even a tiny amount) when the trigger is pulled, the M&P is by definition a DAO pistol, even though in reality it's a deliberately screwed-up, compromised SAO pistol that is easily fixed (with great results). While this hurts the M&P's reputation among gun enthusiasts who prefer a better trigger out of the box, it opens the door to widespread adoption by law enforcement agencies, working around some silly rules and legal issues. This is just a theory, mind you, although it would explain a lot.