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...
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...
I hope to stop using these batteries *way* before there is even a chance they will pop out during flight. They do *not* swell enough to pop out. (Yet!) I know this because I often remove my batteries seconds after the drone is landed and turned...
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...