"So much so that..." stands out as slightly different from the other two.
"To the extent that" and "to the point that" both suggest that there is a specific point at which your statement became true. "So much so..." suggests that there is perhaps too much of something, which might be better in the context of your example where something is out of control.
As to which is more 'natural' - to me, they are all equally natural and commonly used. I wouldn't call any of them 'formal', but they all indicate well-thought-out speech. If you were speaking extemporaneously it is far more likely you would begin by saying "I spend so much time online that..."
I should point out though that there is an issue with all of the examples you gave. You say "I can hardly control how much time I spend online", so what are any of the 3 options actually referring back to? It doesn't make sense to talk about 'to what point', or 'to what extent' you do not do something. Likewise, you can't say there is "so much" of something if you "hardly" do it.
I think you mean to talk about the 'extent of', or 'how much' time you spend online. You need to rethink these sentences. An example of a correct construction would be:
The amount of time I spend online has grown out of control...
Then you could use any of your 3 suggestions and they would refer to the extent of the growth, or how much time you spend online.
