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The Gen 4 housing will not fit in a Gen 3 so you'll have to pull the ejector and install it in your housing. Does your ejector have the number 336 on it? Also, you said you were having failures to extract and that will not be fixed with a new ejector.
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I watched a couple of YT videos on this, but it is not difficult at all. First, when you get the new Gen-4 trigger housing, take a small flat blade screwdriver, or similar tool, and push the back side of the 30274 ejector forward to get it started. Next, take a pair of pliers, with electrical tape on the jaws, if don't want to scratch the ejector, and grab the front part of the ejector and pull it out. Next, take a punch of correct size/Glock tool and remove the rear pin in the frame. It moves pretty easy. Next, push the Gen-3 trigger housing upward just enough to see the back side of the ejector. Take the small flat blade screwdriver, or similar tool, and push the back side of the 336 ejector forward to get it started (just like when you removed the 30274 ejector). Next, take a pair of pliers, with electrical tape on the jaws, if don't want to scratch the old ejector, and grab the front part of the ejector and pull it out, all while you secure the trigger housing as it is pushed upward. Once you get it started moving with the screw-driver it pulls out pretty easy. You may have to keep upward pressure on the trigger housing, but you don't have remove it completely; just push it up far enough to remove and replace the ejector.
Next, keep upward pressure on the rear trigger housing, insert the new 30274 ejector into the slot and push it all the way in; you can get it most of the way in with just hand pressure. I used the plastic handle of the small screwdriver to push it in until it bottomed out in the housing. I've done 4 of these already.
Good luck!