Ok. Now I can finally speak from experience. Actually shooting all 3 in a .45. I would pick any of them in a heartbeat. Now just as far as my personal preference. I was a huge Glock fan. And I mean huge. So this will kind of show my opinions.
Taking the #1 spot. Suprise suprise...the M&P. That's right the m&p. I love the feel of the trigger. The comfort of the grip. Over my ruger this is the best shooting pistol ever. Easy to take apart and maintain which is something all 3 have in common. Plus with the gun being thinner than the Glock and XD it makes concealing more easily. For me this weas definitely an easy pick. I like the ability to see if a round is chambered by looking at the top of the slide.
#2. Tough choice but I am leaning more towards the Xd just because it feels alittle better in my hand. Accuracy was slightly better. I do like the ability to see if a load is chambered on the xd. Now I always double check but I do like the ability.
#3. The Glock. It is a nice gun dont get me wrong. But the comfort level is just not there. Definitely better with a hogue rubber sleeve on it. The way I hold a gun it made the accuracy slightly worse with the glock than with the other two. Its still a good gun and I wouldnt be mad if it is all I had.
Of course these are just my opinions. I can name others who feel the same after shooting all three.