You are having a little problem with your terminology here...
- Decoupling caps are the ones that are wired across the DC supply rails, they provide a power reservoir and reduce the impedance of the supply. If connected backwards, these will normally blow.
- Coupling caps are used to pass an AC signal (for instance audio) between parts of a circuit which have different DC level. These are normally in high impedance parts of the circuit and reverse polarity will normally just lead to signal distortion or biasing issues due to reverse leakage.
It's important to get the distinction right.
It sounds as if you are talking about Decoupling capacitors. Failure time is indeterminate - although it sounds as if you have already suffered a 66% failure rate. The third capacitor
will fail too at some point (unless it has poor solder joints or something). I think you will find that if you remove and test it, it will not be in "working condition", it just hasn't blown up yet. Hot swap might lead to a faster voltage rise time but this is probably irrelevant.
EDIT: If the capacitors are in different parts of the circuit, or you are not sure if the polarity is correct then you should measure the polarity of the voltage across the cap and compare to the orientation. It may be that the PCB silk screen is incorrect.