Random means random.
You could find all five bosses/items in dungeon 4. Or you could find them all in dungeon 5 or 6. Or, far more likely, you could find some in each of dungeons 4, 5 and 6 if 1-3 were empty.
I just cleared out Dungeon 1 in Act 2 and had no boss at lvl 3. Just so I am clear about your meaning:
1. You are sure that the five(?) bosses/items WILL turn up eventually, even tho' they may not be one in each dungeon. Is it possible that a boss/item may NEVER turn up in spite of getting a specific quest to find the boss/item?
2. Are there only 5 bosses even tho' there are six dungeons, i.e., one quest from each merchant?
Best wishes, SerAris