What does the prefix in- mean here? It usually means "into" or "in", but that doesn't seem to make sense with this word. Surely it can't mean dividing communication into smaller pieces.
Partire in tria means to divide into three things. This idiomatic use of in is, perhaps, the reason it was prefixed to partire to make impertire. When you impart something, you are dividing it into things, whether three things, or two things, or twenty things, for the purpose of divvying it up. If the particular number of things is not what's important, there is no reason to spell it out. You just need to prefix in to your verb and get on with saying what's really important.