As the other comment suggested it's language independent. For me, three characters have enough width to be easy to visually distinguish from plain spaces, and the occasional typo, but still easy to keep track of with multiple indentations.

I do suspect that Fitt's Law[0] applies to the question of ideal indentation width though. In that case there is a trade-off between trying to minimize the number of characters for less distance, since that makes it easier to "hit the target" with your reading saccade, and having enough distance to make sure indentation has a visually distinct gap from plain spaces. However, the sweet spot of that trade-off is undoubtedly very personal and dependent on many factors.

[0] https://en.wikipedia.org/wiki/Fitts's_law

An indentation preference can be largely language independent.