I thought I was pretty clear that I don't know what "threefold increase" should mean. Sounds like "an increase of three times normal," that's normal (x) plus three times more (3x). so x+3x seems like a reasonable definition.
Yeh, this is weird. A "50% increase" is clearly "in addition to", whereas "threefold" reads as "in comparison to". I have no idea how to interpret a "onefold increase".
Like I implied, human language is ambiguous.