The nakaten doesn't map perfectly to any single English punctuation so it requires a case-by-case decision based on the context.
Most of the time it separates words like a space would. This is the easiest and most common occurrence. The other cases can require some tough choices.
For something like Ys I·II an ampersand (&) is the most logical equivalent in English, which is the style the publisher chose for the USA title (Ys Book I & II). And that's how we would read it aloud in English ("Ys One and Two"). Other games do that as well: ファイナルファンタジーI・II アドバンス became Final Fantasy I & II: Dawn of Souls outside of Japan.
But one could also make a case for a plus sign (+) or even a dash (-). I think they're suboptimal compared to ampersands but they are reasonable alternatives to ampersands. The plus sign sometimes gets read as "plus" but also "and". We tend to read a dash between numerals as "through" (representing a range) or "dash" so I-II ("One through Two" or "One Dash Two") seems a little bit odd unless you know it's just a stand-in for a middle dot. I've also seen people try to use periods and asterisks (I.II) (I*II) but those are just stand-ins due to ASCII limitations and not serious solutions.
You've also got the spaces-or-no-spaces question. I'd vote for spaces (I & II) since it looks cleaner and that's what the publishers tend to do.
For this, you've got a list of 3 elements so commas are best. In English we don't often say "A and B and C" but rather "A, B, and C." The final "and" kind of says "this is the last element in the list" but it's not necessary in non-speech contexts like titles so "A, B, C" has a more pleasing look and you're substituting the same punctuation each time. It's also a better match for the Japanese since a comma is "silent" and I believe the same applies to nakaten (please correct me if I'm wrong, Fuzzball or Sarami).
What Fuzzball was saying is that semantically it means A & B & C but orthographically it should be A, B, C.
That covers most situations (inter-word separators and parallelization) but you're always going to run into oddballs that are hard to fit into a heuristic.
For example, R·TYPE.
Unless that's just a stylized dash, which is what Wikipedia Japan seems to imply. *shrug*
Be aware there are other uses for nakaten that could require different punctuation but I haven't seen them in game titles yet.