you're right about the gen 4, the 2 and 3 would require gun smithing. They would have to enlarge the hole on the left side of the frame and inlet it to have the release fit properly and flush. Then if you look inside the grip at the front of the mag well from the top you'll see a thin spring steel rod that acts as your mag release spring. as you push on the release you see it bend to the right and there is an off set "V" in the frame to allow it to flex as such. A matching "V" would have to me machined to allow it to flex left instead. And finaly the opening in the right side of the frame is now bigger than the mag button. This excess would either need to be filled with some kind of epoxy then stippled in some way to match the grip OR you could order a gen 4 mag release that would fill the hole (larger button on hte gen4) and allow you to switch it back to right hand again.
In the end the cost of the work would be far more that the money you saved in the "deal" you got on the gen3. If using your left thumb to drop the mag is that importaint to you, you should save a bit more and get the Gen4.
Another option is to train to press a normal mag release with your trigger finger durring reloads. It's not as awkward as one may think and with enough repete use at the range you'll never know the diff...
Example, my HK USP has a somewhat unique mag release for an auto pistol. Instead of a button, its a small button sized lever that hindges downward. This lever is ambi for right or left handed shooters. And altho I shoot righty, I usualy will use my trigger finger to push the mag release on the USP.