"as far as" vs. "so far as" vs. "in so far as"

Are these sentences the same?

1. As far as I know, he's going to Chicago.

2. So far as I know, he's going to Chicago.

3. In so far as I know, he's going to Chicago.

I think that they are the same in meaning but differ in formality. I think the first one is formal, the second one is informal, and the third one is very formal.

Solution 1:

As far as I know, he’s going to Chicago.

So far as I know, he’s going to Chicago.

These are identical in meaning and register. They differ only by dialect or individual speaker preference. According to Google Ngrams, they are approximately equal in popularity:

In so far as I know, he’s going to Chicago.

This is a less popular variant, which to my ear sounds slightly more formal simply because it’s wordier. In American English the spelling is insofar, meaning “to such an extent”.