Is it appropriate to wear a 3 piece to a wedding as a guest for evening invite?

To be honest, it would seem a bit OTT to me. It doesn't matter that you were invited to the whole do but can't make the day, the fact is you won't be at the day and that's the bit that people are going all out for, only IMO of course. Likelihood is though that, if you're concerned about what everyone else will think, they probably won't think of it at all, unless you bring it up, which would be weird. But seeing as you have brought it up here, personally, it seems a bit like being a full kit wanker without actually playing the game, like John Terry at a Champions League Final...