1 trait per type at a time (for example, nirnhoned on bow).
Up to 3 types at a time (for example, bow/inferno staff/shield).
Yeah this.
Back at launch (or maybe it was in the beta), I kinda remember there was a trick where you could start multiple researches of the same item and get the same research time for each, IIRC. So there's not a restriction. You can research three items at once, but not three different traits on the same item.