How did Kakashi actually get the Mangekyou sharingan?

[Multiply]

He had MS as a kid, then he had 3 tomoe in part 1. Proof that he can switch.

yes but why couldn't he use MS in Part I... Proof of my claim is when he fought Itachi, any normal ninja that has MS will use MS against another MS... Yet you're implying Kakashi deliberately didn't activate MS because he didn't feel like it? He can't switch from 3 tomoe to MS willingly. Something during time skip made it switch to MS. It hasn't been 3 tomoe since.

 

[Crossroads]

The most plausible reason is that he knew, but didn't use it due to lack of stamina/chakra reserves and control.
Years invested in becoming a better shinobi, he probably became strong enough to handle using it without killing himself.

 

[mafo]

Kakashi is very tactical he never reveals all of his skills...not even to his friends. I say he knew about it but never said anything since he knew he could not use it then. Like when he first use kamui on deidra, no one even seen him try it once.