The expectation as to how the actual meta keyword value on the page should look like is "keyword1,keyword2,keyword3" (comma separated values with optional whitespace). See http://www.w3.org/TR/html5/document-metadata.html#standard-metadata-names

In terms of EPiServer this becomes dependent on the actual templates / page types used.

The Alloy templates use their own String List property for meta keywords.

The String List uses newline as its separator, so the expectation is that the entered value has one keyword per line.

However, writing multiple comma-separated values on the same line will also result in the expected kind of output, although at runtime on the server side the actual list only has one element (which happens to contain commas). Having values like this could become strange if you were to introduce some form of fancy list editor instead of the text box.

In short, one keyword per line is intended but either of your two first suggestions would also generate valid output. (Do note that this answer is based on the assumption that meta-keyword handling matches that in the Alloy templates.)