I think 1. Dex 3, Str 3 is the same as 2. Dex 3 and Str 3, the serial comma being a stylistic tool: you need both Dex 3 and Str 3.
Even though the serial comma could separate items that have an "or" relationship as well, the usage in the core nWoD book very strongly implies an "and" relation. For example, it would be rather weird to suppose that for Fighting Style: Kung Fu (Strength 2, Dexterity 2, Stamina 2 and Brawl 2) you'd need either Str 2, or Dex 2, or (Sta 2 and Brawl 2). Obviously, you need all of these to be able to pick this merit.
Also, I haven't found any Physical Merit in the core nWoD rulebook that had a prerequisite listing using only serial commas without an and inserted before the last item. Serial commas seem to be used only when a prerequisite list has more than two items (of which the last one is always separated by an and).