I’m not sure I hear people call MacOS X 10.10 “ten ten ten”. I think I remember them calling it “ten ten” verbally.

So you’d say “MacOS ten ten”.

At least that’s what I’m used to, it is entirely possible that’s what other people said and you would write it that way. No one wrote “MacOS X.10” or “MacOS X .10” but they would write “MacOS X 10.10”.

So yeah it’s all a bit of a mess. There’s a reason people often use the name of the release, like Snow Leopard or Tahoe, instead of the number numbers.