During the first several years of M&P pistol production it was very difficult to find any reports of extractor failure/breakage. One time I was told that the factory had only gotten 1 or 2 guns back with broken extractors in the first several years of production.
The M&P I-beam extractor was one they were able to use across the whole model line in the original M&P 9, .40, .357 & .45 models. It was overbuilt on the hook, which we were told had been done so the shoulder ahead of the hook would resist damage if someone had to perform a failure-to-extract drill that required running the slide forward to attempt to recapture a stuck case. (No, we were also told that the company hadn't changed its recommendation of directly loading the chamber without using the magazine. They just wanted to give LE users a bit of extra strength in that regard, in case needed.)
I learned of 2, perhaps 3, revisions over time to the M&P standard extractor.
The first one I was told about involved making the hook edge a bit "sharper", and giving it a bit more rearward rake. It was intended to help give the hook a better purchase on case rims if someone was using "hot" ammo.
The next one I saw during one of the armorer updates, when we were told they made the hooks a little longer. The revised extractors shown to us in the class had an etched "L" on it (scribed by hand, where it couldn't be seen when installed).
Then, just a little while later, when I was ordering some spare parts, I noted the new M&P extractors included the name "Phillips" (sic?) in the part description. When I called and asked some about it, I was told it was the latest revision, but they couldn't give me specifics of what had been revised. I later wondered if the earlier extractors with the hand-marked "L" I'd seen in the class, said to be brand new, had been an early batch of this current extractor, but I never called to ask.
Now, last I knew (I'm due to need a 6th M&P pistol class in another year or so, and a second Shield class), the Shield 45 uses a unique extractor. It contains a lengthwise hollow cavity in which an additional plunger and spring are nestled, captured by the front of the cavity and indexed off the extractor pin.
Also, while I haven't yet done armorer class on the 2.0 models, I was told by someone at the factory that the 2.0 slides are a little different for extractors, and there's a special boss to prevent extractor mix up. Have to get the details next time around. There aren't that many 2.0 guns in-service at my former agency (less than 100, I think, as the main issued M&P's are still the late production "1.0's" in 9, .40 & .45), so it's not like I need to update on the 2.0's quite so soon.
What's interesting is that in the last current parts lists for the M&P's and the Shield pistols, there isn't a "recommended" service or replacement interval for the extractors. The locking block and ejector are recommended for service replacement at 20,000 round intervals, but nothing listed for the extractor. They've been found to be a pretty durable part in exhaustive factory testing in the last 13 years of production.
Now, if the extractor breakage is happening to the OP in 3 different M&P's, I might be inclined to look more toward the common factor (extractor batch manufacture) than the 3 slides. The extractors are MIM, so they're made in batches.
If the customer service person at S&W wasn't interested in having the broken ones mailed back (or preferably the guns, under warranty?), it might be because he/she didn't think the assigned person who receives broken parts would be interested. (I spoke with one the CS contacts for LE several years ago, and he told me one of his jobs was to be the guy who received all returned broken/damaged parts, to inspect them and decide if they needed to be kicked up the line elsewhere for further examination.)
Might also be because they'd already identified that some production batch in which they'd received owner/user feedback of a few that didn't withstand the normal heavy usage. Dunno. Never say never.
Now, when I attended one of the early M&P field armorer classes ('07) we were required to buy a factory Go/No-Go Extractor Bar gauge for .40 S&W so we could check extractor fit. They later dropped that requirement. No more Bar gauges or fitting. Just the plastic pistol tool kit.
As a matter of fact, that change came along before they even produced extractor Bar gauges for armorers for the 9/.45 slides. I know, as I'd kept calling, wanting to get a Bar gauge for each M&P caliber to add to my armorer kit (I have them for all the 3rd gen calibers and the original SW1911). I kept being told they were studying it. One day they finally told me that it had finally been decided that manufacturing methods and production tolerances we now so close and uniform that armorers wouldn't need to "fit" M&P extractors, like had been needed in 3rd gen and SW1911's.
OP, if you could part with one of your 3 guns for a few weeks, to have it returned under warranty, and send a letter detailing the round count and breakage, it might be interesting to see if they did anything more than replace the extractor (and spring).
Never say never.
Just some thoughts.
