Yashica fx3 battery door

The FX-3 was manufactured by Cosina for Yashica during the cost-cutting years, and they used a plastic battery canister that's threaded on the end to accept the matching threads on the plastic battery cap. The cap itself also acts as the stacking holder for the 2 cells used to power the electronics. If the batteries swelled and/or burst, the entire cap piece can get lodged into the canister from the leaked goop that eventually turns solid.

Some camera baseplates used to be threaded for the cap to screw into and removing that plate would bring the cap with it. That won't work with the FX-3. It's plate is softer and too thin, and won't hold a thread. Since both the battery canister and cap are softer plastic, it might be possible that the cap got cross-threaded somehow and jammed, but that's less likely than the corrosion explosion from one or both cells having leaked and dried over time.

I don't know if all of the FX-3 models had the same cap, but both of my Super 2000's have caps with a small hole (maybe 1mm round) at the bottom-center of the 'coin slot' used to unscrew the cap. I can clearly see the silver metal of the battery showing. If yours has the same hole, it should be obvious if there's a set of cells already installed.

If you have a #00 JIS cross-point screwdriver, you might be able to remove the camera's baseplate and then unscrew the 2 screws holding in the battery canister so that you can partially remove it and try to clean it with cider vinegar on cotton swabs. I wouldn't try to loosen/remove any battery gunk while it's still screwed to the body. Vinegar can still cause damage to camera internals and lubrications. I'm saying 'partially', since it's likely that there will be wiring still attached to the canister so you may not be able to entirely detach it. If you can get some vinegar to seep into the holes/crevices on the cap and canister, it may loosen the corrosion enough to normally unscrew the cap. Flush the vinegar off with water on a swab to stop any chemical reaction once done with cleaning.

If cells did burst and the leak was severe enough, the corrosion can travel up wires or metal connectors and cause damage beyond the battery canister. There's no guarantee that cleaning out the junk will give you a working camera when done. If the damage reached a circuit - or if the leak caused a sustained short - the electronics might be damaged or fried. There's also a spring plate in the canister that might be corroded and no longer workable - as a spring or a conductor.