Source: direct literary origin of "three things a Buddha can not do"?
So far as I can tell these "three" are attributed to a Chan monk named Yuangui 元珪 (date 644 through 716).

An example story to illustrate  the first ("can't destroy karma") is his being unable to stop a king destroying the Sakyamuni clan.

I think the second means "can't save people who refuse to relate with him". The example story is of an old woman who disliked him when she saw him. He used his supernatural ability to miraculously show her everything, and she disliked that too. But she liked or had an affinity for Ananda when she saw Ananda, and so learned from Ananda instead.

The third is explained as something like, the people are (at least in part) saved or not saved through their own effort (their own understanding etc.).