Translation:Matěj asked us the day before yesterday whether you had ever been married.
Předevčírem is an adverb so there is no declension happenning here. It is more like "přede" (before) and "včírem" (that does not really exist separately). Basically it is "přede" + "včerem" as if "včera" was a noun. But it is not, so do not get confused about that, it is also an adverb.
The word has just one form and you have to learn it. The form is: předevčírem.
Včírem on its own can be actually rarely met in old texts, but I suggest ignoring it.