As others have advised, these are done. Properly dispose of them.
Any swelling AT ALL indicates hydrogen gas is being produced during normal discharge. This only happens when one or more cells is compromised. There is a very real...
DJI batteries are known for swelling based upon age rather than usage. The alarmist posts above are well-intended but inaccurate. The biggest concern with DJI batteries that swell is a potential disconnection during flight when they heat up...
I think it's better to get them down from 100% sooner rather than later. When they are at 100% the chemical reaction that degrades them happens faster. 3 years sounds about right for a battery that was kept at full charge to start bulging.
Also...
DO NOT CHARGE THEM! You can burn down your house very easily doing that!!!
Also if they swell too much in flight, They wont come out of the Drone and then you will have to wrench them out.
After flying all of my drones on day 1, all of my batteries are technically "used." Generally if you buy a used drone, you get a used battery too. Unfortunately, pretty much all the drones flying out there at any one point are likely over...