They gate check them. It's just a customer-unfriendly on-time strategy, most noticed recently on US flights.

If you're trying to get 3 bags on, then I have little sympathy for you. If you have 2 compliant bags and are the last one on an E70/E90 for example, which always run out of space, there's no way they should toss you out to fend for yourself. And I really don't think they would.

I prefer the GA's who announce that the flight it full, the plane had limited capacity, that historically the last boarders will probably have to check their bags, and request anyone who can bring their bags up in advance to speed everyone along. Others aren't quite as nice/proactive.

Personally, I've never seen TSA care the slightest bit about how much you are carrying, as long as it fits through the scanner. I've seen items that I know will never make it as carry-on and they just focused on scanning it.

If they said something to me, I'd first ignore them. If they refused to let me continue, I'd get a supervisor and ask what basis they were using for refusing to process me. Then I'd tell them I had arrangements to not violate the airline's policy and they shouldn't be concerned. I've never tried to carry more than the allowed items, so a bit of a moot point though.