As I understand, the expiry date is set by the degradation of the naloxone in Suboxone, as opposed to the buprenorphine. Naloxone is very unstable in the film formulation of Suboxone, degrading rapidly to a large number and high levels of impurities. However I haven't seen any good evidence that these impurities are dangerous (although it really hasn't been studied enough either), I think they are just more concerned that there will be less naloxone in old strips. I'm not sure if the buprenorphine could have degraded somewhat over 3 years, I think it should last beyond the expiry date but I'm not sure how long. It's also possible that you could have not taken enough or took it too soon after taking opiates.